It is not an error to share a Sequence in different mappings but with different Values for increment_size.
This reliably leads to insertion of duplicate keys.
All in a rather tangled fashion since duplicate keys are generated for one Table, when you would expect it to at least be consistent internally for one mapping. But the mapping with the lower Value for increment_size probably resets the Generator and then the larger one steps back.
Do a Validation phase when reading the Mapping and check for parameter consistency between all Sequences that are shared.
Possibly give an explanation in the error how to find out the increment_size in the database and to set it in all mappings to that value.
New Style Sequence Generator is used.