2 Cent Franklin Missing Ink

fount this cover in a lot and the 2 cent franklin is showing unusual un-inked spots all over the stamp, specially in United states postal and left oval with 2.

hoping one of the experts can help me, my questions are

1. what made this happen?
2. is it a DT? or an error?
3. are they common or rare?

This is Washington, not Franklin.

1. This was improperly inked, with possibly either insufficient ink or too dry of an ink mix.
2. Double transfers are when portions or the whole of the design on the printing plate are incompletely erased, a new design is placed over it and two impressions or parts of them are printed together. See:
https://goscf.com/t/11164
This is not considered an error as defined in philately. It is merely a minor flaw in printing.
3. This is common enough, not a rarity at all.