When you see something like this, please raise a moderator flag.
Just explain what you think in the free text, like: "This looks like a sockpuppet account."
This is important, as moderators do miss things.
Moderators have tools to identify and deal with cases like this.
I will not describe the tools here; secrecy makes it harder to play against them.
We can deal with these cases easily and we also have access to higher level support when needed.
Suspicion is reason enough; we can investigate.
If a user posts unwelcome content, you can act as usual:
vote down, vote to close, point to relevant meta or help pages.
But as usual, there is no need to burden yourself.
You can always just disengage and leave it to others, in this case moderators.