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You might want to try giving a little more detail. I can think of a dozen different places it could leak, and a couple of reasons for each place to leak.
If you are referring to the front (input) seal, then "Yes", it's very common.
Some folks try to just replace the seal and nothing else. Unfortunately, if the seal is worn, the bushing is likely worn out too. Just replacing the input seal won't work for very long.
If you have a leak somewhere else on the unit, please give a better description. There are about a dozen 'common' areas of leakage on any GM auto transmission.