Protect against applying the offset more than once

This pull request fixes a bug by preventing the digi accumulators, used by the mixing module, from applying the out-of -time offsets more than once to the same hits.
Note:  This does not fix the more general problem where a digi accumulator and a crossing frame each apply the offset once to the same hit, resulting in a double offset.
Tested with checkdeps -a and the relvals.