Question - What if there are two of the same Sim?

@iamsweetmystery I'm not sure how much this will be helpful to you, but I did a few experiments that may help answer some of your questions.

First, I would recommend that you run a game repair then save a copy of your game f you were having issues:

Second, I would highly recommend that you make a full copy of your game file as a backup before doing any experimenting:

Thirdly, there are a few things I have done in my game saves that may or may not duplicate some of the questions/issues you are having... 

• Personally, when I want to make a new game save, I normally just open one I want to mostly duplicate, then choose the 'Save As...' option, and I give this new game save a unique name and save. Then I go into the Main Menu > Load Game and confirm that the original game save I copied is still there and working correctly. Then I enter the new game save I just made and do any editing of it that I want (removing any households, plowing lots I want to use differently, etc.). As-is this game save will have preserved any households and relationships it originally had, unless you delete or alter those.
• Sims can be moved from one game save into another game save by saving them in a household to your game's library, however, keep in mind that the only relationships that will be preserved with these Sims will be the relationships with those that were saved with them in the same household when added to your library.

Now, with all of that in mind, I did a few experiments...

I have a save where I had accidentally placed a copy of my 'Sim-self' (with a different name, a copy of the same hubby, and different children). So, the Sheri's in these tests are from my original one that I have played for many years with aging off. 

So, in this game save I placed on a lot and one of the Sheri's (Sheri Similator) in live mode. I paused then opened the cheats window/box, enabled testing cheats, and added the following in the cheats window/box:
sims.get_sim_id_by_name Sheri Similator 

Then it gave me this Sims ID code:

If I then went in and changed her last name to Sims in CAS, when I checked, her ID was the same.

Next, I went into the household with my other copy of my original Sim-self. In this household she has the same hubby, different kids, and her ID is different (probably from placing her and her hubby in this game from a household saved into my library). I would guess that when you place them from the library they get a new ID. It also would not surprise me if when you make a copy of the game they may (or may not) get a new ID. I didn't test these scenarios.

If these two Sims met on the street they would have absolutely no relationship that didn't come from any interaction or normal ways of developing relationships. I've never had any issues with this stuff bothering my Sims when I miss it initially. However, it bothers and confuses me, so I make corrections (removing a household, editing relationships, etc.).

I have accidently had two copies of a Sim running around town. I play with Cas.fulleditmode on all the time cause I'm always fixing townies with their odd outfits and purple or green hair. So I just shift clicked the extra sim when I saw him and fixed the name and gave it a makeover.  However you can also replace it with a sim from your library or one from the gallery. Easy to do and it's a quick fix.

@SheriGR - I guess in my question I forgot to mention this is on PS4. No Origin/EA app. No option to Repair. If it was on PC, I definitely would have done those things, but on console I had no choice but to start a new game when I got two Chestnut Ridge Worlds. Thanks anyway. The confusion there was my fault. Sorry 'bout that.

@PipMenace - Sounds crazy and confusing, but just what I needed to know. Thanks.

@ReneHollyDesign1 - Assuming it didn't break my game, this is precisely the solution I had in mind. Thank you. With the answers given by everyone, this gives me some peace of mind. Now I can confidently send Molly back to Sulani with her lovely Vampire Fiancée with no worries. I might just change the Copperdale Molly's name (was considering Milly or something close to maintain some consistency), and my rescued version's wardrobe. Should suffice.

@PipMenace - Sounds wild, and funny. This has happened to me before. Last year, Forgotten Hollow duplicated itself. Obviously, given the time span, there was less to rescue. This time was worse and more existing characters were involved. Some I don't mind deleting the original, like Miko Ojo, since she only has 2 roommates and a backstory I don't care about, or Eliza Pancakes, who only has Bob. Both have moved on to other loves in my game and had one or more children, so mine takes priority. Molly is from a large family, daughter of the school principle, and is kind of an icon of Copperdale school life, so I don't feel right erasing her from existence. Glad I don't have to.

When I first started playing the Sims 4 on Xbox years ago I wasn't used to the household format and I couldn't figure out how to get my groom to move in on his wedding day. So I incorrectly (daftly) added him to the household from my library. So I had one groom and one, well...groom clone. The groom clone congratulated the groom on his wedding and the groom, went home to a different house. I don't know who was more confused, me or the bride! Eventually I figured out how to fix it but I wish I'd known about this forum back then lol I would have had so much help with it!

@simsplayer818 - Hilarious story. Love it. As long as it's not game breaking, it can be funny sometimes when things go a bit sideways like that. Frustrating occasionally, but funny. I've certainly had my share of "why did I do that" moments, so I can relate.