What Is A "Double Sabbath"?
Excerpt from Keep the Feasts by Inawah Yasharahla
A double sabbath is when there is a weekly sabbath and the following day is also a holy convocation.
It extends from sunset Saturday to sunset Sunday.
You will not find this term in scripture!
The term was coined to refer to times when a holy day falls the day after a sabbath.
Since those days follow the same guidelines as a regular sabbath (see Chapter 5 Sabbath Protocols) it is like having two sabbaths back-to-back.
In conclusion, not explicitly stated in scripture but a term coined to mark the occurrences.