Does anyone know if this is common?

It has been known to happen

Sometimes it appears that way because the first church was a "daughter" church to a main one , examples in Bream and Newland; and what I think we see is the record in the smaller church records and then it being added / duplicated in the main church records.

used so many different churches

remember these churches are very close - Drybrook, Flaxley and Abenhall - it they were living in the extra parocial areas of the Forest they would not have a "parish church" as such. And then you have to bear in mind the evolution of parishes, Drybrook was consecrated much later than the older parishes of Flaxley and Abenhall.

see here for a little more help
http://www.forum.forest-of-dean.net/index.php?mode=thread&id=10483#p10519