You can add another Global called Enemy_Sprite and make it random with choose(1,2,3,4...)
Then you make if conditions
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| IF Enemy_Sprite = 1 -> Spawn Enemy1 at ...
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| Else
| IF Enemy_Sprite = 2 -> Spawn Enemy2 at ...
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| Else
| IF Enemy_Sprite = 3 -> Spawn Enemy3 at ...
-------
.....
When does the overlapping occur?