To get duplicates, try this:
var doubles = xdoc.Descendants("PositionDetail")
.Descendants("AccruedInterest")
.GroupBy(item=>item.Value)
.Select(grp=>new
{
AccruedInterest = grp.Key,
Count = grp.Count()
});
Result:
AccruedInterest Count
0.000000000000000000000 2
-883.889000000000000000000 2
883.889000000000000000000 2