Keyboard shortcut conflict causes script line to be duplicated on macOS on Play

[ArdaE]

If I "play" a project while editing a script, the line that the cursor is on gets duplicated. It took me awhile to discover what was happening, as my scripts kept getting duplicated lines (which in some cases simply introduces bugs into the scripts without any error message depending on what line gets duplicated). To make things worse, the line duplication isn't even immediately visible and doesn't affect the current Play of the app (it becomes visible and takes effect when you go back to Godot after running the app, at which point you may be focusing on something else). In the current master build of Godot (currently using commit 94b88c7), "Editor -> Play" and "Script Text Editor -> Clone Down" commands are mapped to the same Cmd-B key combination for macOS (not for PCs). The solution is easy on my end, which is simply to remove the assignment for "Clone Down", but this would introduce bugs into the scripts of unknowing Godot users.

The solution should be to 1) remove the default Cmd-B shortcut duplication outlined above, 2) add a warning to the shortcuts page when a user tries create a shortcut that's already assigned to another command (as a nice to have addition), and 3) have a compile-time check for duplicated default shortcut assignments so this issue doesn't repeat for other commands (who knows, there may already be other conflicts lurking in there).