Mojave v10.4.3 has a rather insidious bug in the code related to Drag-and-Drop. It almost looks as if the event handler is firing twice for one Drag-and-Drop action. This manifests itself as the app appearing to receive two lists of dropped files, instead of just one. This is further complicated by the observation that it only behaves this way for when certain filetypes are part of the set dropped. To date TIF, PDF and DMG files seem to be affected. MP3 and WEBLOC are not. There may be (and likely are) others.
This issue can be a nightmare when developing apps, so I thought it best to post it here. Hopefully, this keeps others from the sleepless nights I’ve had. :>)