Im not sure you can compare mail context with address book context. Sources could be external to roundcube (google, company AD sources, etc), which could be also part of another mail environment in itself. At work I may only have the AD source, but at home I was able to combine work address book with home address book. So if I drag a contact from my company address book to my google address book I probably mean to copy, not to move. Especially if those address books have separate meaning outside of RC. I wouldn't want contacts to just disappear from my (private) google address book just cause I wanted to copy a contact into a work address book.
But I can also see how duplicate contacts could be a drag.
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