I can't see any reason that one character cannot have multiple master writs in their quest journal of the same crafting type at a time. I can see not having literally the identical writ, but otherwise it's just a huge pain.

I just got done doing 72 writs. I got 2 master writs for them. Both were 2 voucher enchanting writs. I got to have the fun of doing one, turning it in, going back to the distant enchanting table, returning all the way back to turn it in again. For 4 vouchers. Yay.

Potentially even more important is people who accidentally accept a master writ on a character who doesn't not have sufficient traits of the motif learned for the writ, and might not for months. In the interim they cannot get a new writ to replace it without entirely losing the initial precious master writ from their journal by abandoning it. I'm sure someone out there has made such a 500k mistake.