Can 2 iPhones have the same imei number?

An IMEI is a hardware identifier. No two cell phones can possibly have the same IMEI. Just like 2 cars can never have the same vehicle identification number, or two residents of the US can have the same social security number or the same driver’s license number.

What do you mean what if someone "has" your IMEI number?

You can't "do" anything about it...

They can't use it. They could try to report it stolen and get the device blacklisted, but they generally can't do that unless they are the account holder.

Why would someone else have it to begin with? Unless you posted it on the internet or otherwise gave it to someone for some reason.

No.