Learn English – “There’s someone’s at the door” or “there’s someone at the door”

sentence-construction

"There's someone's at the door"
"There's someone at the door"

What does the second "'s" stand for, "is", "has", or it is a possessive or third person singular s?

Is it incorrect to say "there is someone at the door"?

Ps. I came across it in Touchstone 2 book. Sceenshot below:

The transcription in the image is wrong.

It's likely an error on the part of the editors.

It should read in one of the two ways:

Now there's someone at the door.
Now someone's at the door.

Combining the two doesn't work.

In either case, the word being contracted is "is".

Leaving it un-contracted is also perfectly fine:

There is someone at the door.
Someone is at the door.