I don't really know if this is the same, but I have all kind of bugs like this. Two that happens refularly:
1) When I select action, ctrl-c, then go to another state, click an action in the state (so I can paste the actions before). When I ctrl-v it pastes/duplicates the action I clicked in the current state, not the actions I selected and ctrl-c in the first state.
This has also happened in the same state. I copy an action in the state and click another action to paste the action above, it now pastes the action I clicked.
2) Sometime when clicking an action in a state it makes a duplicate. Just from no where, just clicking it.
It feels random but I can almost feel it when it's going to happen because it's fairly regular. I now press ctrl-c several times just to make sure I get the correct copy data. It seems to work but could just be in my mind.
Unity 2018.3.12f1 Playmaker 1.9.0.p16