Yes and no.

I'm a professional event photographer and I run into this fairly frequently. I don't work a lot of weddings but I do many conferences and other less formal events where there are often other amateur/attendee photographers during an event.

A skilled professional will be able to maneuver amongst a crowd including a crowd that involves other photographers. For many events, there are many possible photo angles and I rarely run into an issue where I have a conflict over physical space with other photographers.

That said, my contracts always address the issue of exclusivity / priority. The contract specifies whether I'm the exclusive photographer, or perhaps it will specify that I'm the sole professional photographer but there is an understanding that other persons may have cameras. I have a discussion with the client and we come to an understanding that if needed, I may ask attendees or amateurs to move so that I may perform my job. Being able to do this discreetly and with some tact is a great client relations skill.