OK I see the source of the confusion. I removed the check from "Allow Multiple Instances..." in preferences. Now I get only one occurrence of BIAB and each time I double click, a new file replaces the prior one in the same single occurrence of BIAB. No more multiple program occurrences.

This was apparently a problem that I didn't know I had since I never use this workflow for opening files. It appears that his options is set by default in 2021. Not the way I would want it. Is it possible that BIAB 2020 did not have it set as the default? This would explain the OPs differences between 2020 and 2021.

Goldy, can you let us know how this is set in your 2020 and your 2021?

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BIAB – 2025, Reaper (current), i7-12700F Processor, 32GB DDR4-3200MHz RAM, 1TB WD Black NVMe SSD, 2TB WDC Blue SSD, 1TB WD Blue, 2 TB SK NVMe, 6 TB External, Motu Audio Express 6x6