Understanding 「(verb-たら) + (verb-past tense) + で」

Question

Consider

休んだら休んだで、たくさんやることがある。

From having taken a break, I have a lot of stuff to do.

or

一段落したらしたで、次の難問が待っている。

Just from following down one step, the next difficult problem awaits.

What is going on here with this pattern and usage of で?

1. Usage of で. I know that で is following a clause, not a noun, but does it have any relationship with the "within-the-bounds-of" meaning of で? Perhaps there's an implicit nominalizing の before the で that's being dropped? Or is this で the て-form of だ? Or is it just a completely separate particle with no connection whatsoever with these other two things?
2. Repeating verbs. The overall structural pattern here is (verb-たら) + (verb-past tense) + で. Why is the verb repeated twice in a row? And why must the second instance be in the past tense?

Answer 1

The basic meaning of this construction is well covered in another answer, so let me focus on what's directly asked in the two questions.

1. Usage of で: At least one dictionary explains this as a rare conjunctive-particle-like usage of で as a "scope/condition" particle. It's not uncommon for a particle to exceptionally take the dictionary form of a verb in certain constructions and set phrases (eg 察するに, 見るからに, やるにはやるが, 飲んだは良いが, 目指すは). At least some of them are related to so-called zero-nominalization or 連体形 as a noun, but I'm not sure if this is also an example of it. Practically speaking, I think it's best to memorize this as a pattern.

2. Repeating verbs: This pattern carries a sense of "what's done is done (and can be forgotten, but...)" or "if you did it, you did it (and that's the end of it, but...)". This pattern implies that another important issue will soon follow.