For my Mum with dementia, there are two of everything and everyone. Is anyone else dealing with such a thing?

When my mom, with Alzheimer's, lived with me, she thought that I was a nurse, a man, or the "other" Andrea.

Now she knows me. But, she thinks all of the nurses in her TCU are "Courtney," my sister.

My late FIL confided in us one day that he had another home in the neighboring town just exactly like the one we were visiting him in.
He went on to say it was furnished exactly the same.
Later my husband took his dad for a ride and asked him to take him by his other house. It fell apart at that point. His dad became confused and they went for ice cream.
Several months later he was at our home as he had been evacuated due to a hurricane and he told me he really liked our house and he liked that it was exactly like our other house. It made it easy to get around, he went on, because we had everything in the same place.
Later, when a grandchild left to go get needed medication, he came to me and said, “I know grandchild went to get my things. That’s good, he said. I just wanted to ask..which house is she going to get it from. House A or house B?” “House A” I said.
“Oh, that’s good,” he said.
He did not have Parkinson’s. He had recovered from a fall five years earlier. We felt it was dementia related to brain injury from that fall. He was not confused about people. He wasn’t angry or upset about the two houses. He seemed to think it unusual himself but he was very serious when he brought it up. We didn’t try to convince him he was wrong.

Retrored: Although your mother suffers from Alzheimer's, her hallucinations may be (somewhat) similar to those with Parkinson's. Perhaps she should see her neurologist.

My mom was engaged to a man for 12 years, then met my father and married him 6 weeks later.

Although my dad was her ONE AND ONLY husband, she swore years later that she’d been married twice.

My husband has Parkinson’s with mild dementia and has had similar issues. The neuro-psychologist diagnosed it as “reduplicative paramnesia”.

After my adopted Mama had her vascular stroke and the dementia that followed, she said there was a strange man that visited her every day at the nursing home. We were concerned until we figured out it was her husband!

To the day she passed, she swore that man was not her husband, although he was very nice.

She was very miffed that her (real) husband didn’t visit her once in the four years she lived at the SNF.

Seems to be a clash between the reality of "today" and her memories that masquerade as "today." Reorienting her to reality might work for a short while but may also be very upsetting. Go with the flow and be kind.

Yes. Look up Capgras syndrome. It’s horrible. Good luck💜

I know exactly what you're talking about. I've had several care clients like this. They would complain about me to me and not realize.
Most of the time I would just speak plainly and explain in simple terms that I'm the person they're complaining about and saying terrible things about. There is no double of me somewhere else.
If they were persistent in the delusional nonsense it doesn't work to keep trying to explain over and over again. Sometimes I would just tell them that nobody wants to hear their nonsense and that I'm there to help them and nobody else is helping them. This usually shuts down the delusion. Often times you just have to ignore them if it's safe to. When a client would start up that thread again about the "other" the "nice one" or the "mean one" I would tell them calmly that I'm not discussing that nonsense with them and then say no more.
This kind of behavior is a kind of dementia loop similar to repeating something over and over again. They get fixated on the subject and don't let it go. You have to break the loop. Sometimes distraction with food or an activity works. Sometimes it doesn't. Ignoring and refusing to give any attention to the subject of the loop really works.

Yes! My mother thinks there is another Becky, and she wants to go back to her "other house", which I interpreted as being the old farmhouse where she grew up. Her current house (where she and the family lived for the past 55 years) seems to look foreign to her, and she often says things like, "where do I sleep?", "can anyone just come in this house and rent a room", "these houses are not as good as my other house". I attribute it to some kind of delusional thinking, or possibly loss of recent (55 years) memory! How I usually handle it is to just go with the flow as she is talking about things, and depending on her mood, try to steer her back to reality.
It is comforting to know that others have similar experiences.