The writ you purchase lasts until midnight of the purchase date, and only the last writ you purchase is in effect.
I.e., if you purchase two writs, only the second one works.
Let's correct afew stuff here.
There's two groups of writs - invitation and challenge, both pretty independent of the other. Silver and Gold invitations stack, but the basic does not with Silver. Challenge writs do not stack at all(so far at least - i'm only a Duke, so maybe a Grand Duke can clear up whether the next set of Writs stack)
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