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Based on the information provided by OP in this thread I believe @smbrennan has a multi-user (network) seat of AEC Collection, so only 1 seat should be consumed when he runs Revit 2019 and Revit 2020. If two are consumed, I believe this issue might be the reason:
https://knowledge.autodesk.com/search-result/caas/sfdcarticles/sfdcarticles/License-Error-4-132-0-or...