check if the object is already parented to said deformer before trying to add a
new modifier
This should help reduce the number of cases where users inadvertantly end up
creating multiple deform modifiers pointing to the same object, which has been
known to be a cause of "double-transform" artifacts.
Note that this is only able to detect these cases by checking if the parent
object is selected, so this will only really work for the Ctrl-P shortcut where
you have to select both objects first. However, it shouldn't be a problem either
in the Outliner (drag and drop), as the object probably won't be a child of its
parent already if you're doing this.
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