We had a boys name picked for ages, before we were even pregnant, and one of DH's cousins used it. We were annoyed but given we hadn't told anyone we couldn't really complain. Made finding a boys name we liked hard, but we did.
The name we both love for a girl is a cousins' middle name, and we mentioned that we liked it and would like to use it. The mother said we were silly for thinking we had to run it past her, and the little girl thought it was awesome the baby could have her name