It's pretty rare, since getting both systems to work reliably is difficult, and introduces some additional mechanical complexity - the FN Minimi (adopted by the US as the M249) has such a system, but it's apparently not very reliable when feeding from magazines. The Soviets had a prototype with a dual-feed system, but never adopted it.
Are there many guns with dual feed options?
It's pretty rare, since getting both systems to work reliably is difficult, and introduces some additional mechanical complexity - the FN Minimi (adopted by the US as the M249) has such a system, but it's apparently not very reliable when feeding from magazines. The Soviets had a prototype with a dual-feed system, but never adopted it.