So, the other day I made a new custom hood with Pleasantview, Riverside, Desiderata Valley, Bluewater Village and Downtown as subhoods. My idea was to let all Romance sims free to create chaos! Every sim with the Romance aspiration who is married or just living with other sims would get divorced and/or move out on their own. I started to set this mini-megahood from Pleasantview and the Pleasant family. When Mary Sue and the girls left the house I had Daniel to hire Kaylynn so that the cheating scenario would happen (now that I think about it, the "hiring" part was unnecessary because Kaylynn was already in his relationships panel, so he could just invite her over!). Now, I know that when I use meetmetotheriver's Maxis hoods as subhoods, the names of the townies and service sims are randomized and I have to change them back to the names we all know.
When Kaylynn arrived, she had a random name so I used the SimManipulator to change it. She and Daniel woohooed (twice, lol!), I got the chance card for Mary Sue and I got her demoted so she came home, she caught Daniel cheating and she divorced him. Daniel moved out and I placed him in one of the condos next to Don Lothario. I continued with the other families in Pleasantview (changing clothes, hairstyles, getting jobs for the unemployed sims e.t.c.) and at last I placed the Burbs and the Oldies. Meanwhile, I was watching the new Let's Play by Pleasantsims and I realized that, although I have made Bella playable in a lot of my hoods, I had never made Kaylynn playable so I thought I'd do that too. I think that, especially Bella is "needed" for my "project" because she has the Romance aspiration.
So, I tried to summon Kaylynn in a household and make her playable using the SimBlender (because I hadn't done that before with the SimBlender, either). The summon menu came up and (in plain horror!) I saw that there were 2 Kaylynn Langerak there to summon. I went to Don's household, knowing that he also had an affair with Kaylynn, to check how... many of them he had in his relationship panel. He had only 1 and he had a very high relationship with her (crush, love and everything!), so it had to be her.

Can anyone tell me what to do now? Is there a way to fix this? Is my new hood doomed?
Thank you in advance for any suggestions!

Do you use empty templates? (If you don't know the answer, it's probably 'no.') If not, this is normal. I ended up with a duplicate set of Pleasantview townies myself because I forgot to swap out the templates prior to creating Oakbrook.

Just figure out which Kaylynn is the one you want to keep. Then give the other one a makeover and change her name with the Sim Manipulator.

I was going to ask...what happens when you teleport both of them to your lot? Do they look the same and when you make them selectable do they have the same personality?

Thank you so much Bulbizarre
I've read about clean templates but I'm not sure how to do that. I just created a new empty custom hood and I set some Maxis premade hoods as subhoods (the ones by meetmetotheriver!).
Rosawyn, that's exactly what I ended up to do: I teleported both "Kaylynns" to the Oldies' house, I made them selectable to check them and guess what... One "Kaylynn" had romantic relationships with both Don and Daniel, she had memories of woohoo with Daniel and winning the fight with Mary Sue (yeap! when Mary Sue caught Kaylynn and Daniel in bed, she attacked Kaylynn not Daniel, probably knowing she didn't stand a chance to win a fight against Daniel ), she also had Angela, Lilith and Mary Sue in her relationships panel and a slightly different personality from the other "Kaylynn", who had no relationships at all and just one memory of going to college. "Kaylynn" with relationships was a little less Outgoing and Playful than the one with no relationships but they looked exactly the same!
So, I kept the one with memories and relationships and I changed the other's name and made her blonde. Coral said "goodbye" to the "other Kaylynn" and the "real" Kaylynn after moving in with the Oldies, she went to the computer to "find her own place". When I placed her to her own new house, I checked the teleport menu of the Sim Blender. The "other Kaylynn" was one of the service sims under her "new" name! I think the problem is solved but I'll run Mootilda's Hood Checker just to be safe!

Thank you!

Those are the clean templates we're talking about. It's important to disable the standard npc/townie pool when using them (or any time you don't want the same boring group of townies and npcs wandering around). You probably have two "Gordon King"s as well; I'd check that.

If you don't want to restart the 'hood, check this thread out: http://modthesims.info/showthread.php?t=582165

Rosawyn: I checked my hood and I don't have other duplicate names, in fact I don't have even one "Gordon King"... I know that doesn't mean anything and Gordon's character might be two other sims but I can't go through all the townies and the service sims to find out. How do I disable npc/townie pool? The only thing I do (in all my hoods) is just ban townies and downtownies from all my lots (residential and community) with Visitor Controller because I simply don't like them and I prefer to play with only my playable sims. What am I doing wrong?

Bulbizarre: Thanks for the link, I read through all of it but, like I said to Rosawyn, I find it to tedious and time consuming to go through all the townie/service pool of each and every one of my hoods and rename/give them make overs. I use the Hoodchecker and fix everything the tool finds that needs fixing, so are my hoods safe or are they going to collapse soon?

The second set of Pleasantview townies had randomized names in my 'hood. SimWardrobe's Visual Teleporter is a good way to check at a glance whether you have clones.

Don't worry about it, honestly. I have Christianlov's Gussy Up plugin so I'm able to do makeovers on the spot when I find a doppelganger.

There may be better tutorials but this is the one I had bookmarked: http://www.woodensimolean.com/node/43

This is what my folder looks like right now:

I can confirm that I do not have any of the default npcs or townies wandering about in any of my hoods, so I must have done it right.
Please note that you have to disable the default townie/npc pool before you start playing a hood; it won't do anything for hoods that you've already loaded up even once.

Since it sounds like you don't want any townies at all ever, I recommended a full set of no regen/respawn hacks (especially notownieregen):
From http://www.moreawesomethanyou.com/ffs/al/hacks/ (assuming you have AL, otherwise you'll want the directory for your most recent EP )
- notownieregen, nodormieregen, and nossrespawn to prevent the game from spawning townies, dormies, and Secret Society members respectively.
- nostrayrespawn if you don't want the game to continually spawn stray dogs and cats
- antiredundancy if you only want the game to spawn one of each NPC rather than the standard 3 Maids, 3 Gardeners, 3 Butlers, etc.
Cyjon's There Can Be Only One (Professor) if you'd rather have only 1 professor for each major instead of the default 2

A couple of notes about the no regen/respawn hacks... (as someone who's played with these quite extensively)
- With nodormieregen and nossrespawn you will get exactly one dormie who is an SS member
- With nostrayrespawn will get one wolf but no Pack Leader
- With antiredundancy if you for any reason happen to have any given NPC "busy" (for example, invited over the Mail Carrier) the game will go ahead and generate a second one for any event where that NPC would normally be needed (for example, mail delivery time)

There goes my plan of trapping the mail carrier in a dungeon and never receiving bills again.

There's a Pleasantview template on meetme2theriver's post that doesn't have any townies in it, using that should result in additional Kaylynn Langeracks not being added to the neighborhood. I installed that version and I actually have zero Kaylynn Langeracks in my Uberhood (I attached Pleasantview as a subhood and my original Kaylynn Langerack got renamed to Elizabeth Robbins, and I decided I liked that name better, so I let her keep it). I do not have any no respawn hacks.

My Kaylynn was named Cherry Custer, but I changed her last name to Custard.