Book I bought at an airport

When I was younger, maybe 15-25 years ago, I bought a book in an airport. I lost the book before I finished it, but I cannot remember what book it was and don’t have too many details.

The book was about an undercover operation. There was a twin who was in the police/fbi/cia who died or went missing, so they had his non police/FBI/cia twin go as him undercover to investigate.

I remember there was something with a woman in the town and the twin. Maybe a relationship? I think I remember the cover being blue. I have no other memories of it…