You need to provide exact steps and specify whether Ripple Markers is turned on. If it is, this might be by design.
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What you wrote is not very understandable. Also, steps to reproduce it are needed. Think: how can I make this a useful bug report instead of vague descriptions of moments of confusion or surprise?
- I would like to consider overlap of markers an error
Not a bug
- File->export->markers… do not generate the correct file
Yes it does, but it does not export markers with a duration > 1 by design (documented here). The format does not support duration and typically these are used for special purpose like export.