If you add print() debugs. Does the function get called twice on loadup like you say?
Is the parent element still the MinimapCluster?
If I do
Lua Code:
/run print(DurabilityFrame:GetParent():GetName())
I get UIParent. (Which is correct because the parent in FrameXML\DurablityFrame.xml is UIParent)
If I had to do sth like this the first thing I do is to get the latest FrameXML files.
I then use TotalCommander (Windows) and to a text-search (Alt+F7) in the FrameXML folder for the string "DurabilityFrame".
The only occurence of DurabilityFrame:SetPoint is in UIParent.lua.