How to create a list that dynamically updates based on other selections

Hi, I'm trying to create a "top twenty" priority list for each of the jobs in column B below.
I would like to create a data validation rule that allows a user to assign a number 1-20 in column A for any of the jobs A-Z in column B, but I don't want the user to be able to assign the same priority to more than one job.
And after the user reassigns the priorities I want to be able to re-sort the jobs in order of priority.
Is this doable?
Thanks,
-Dan

PRIORITY JOB
1 JOB-A
2 JOB-B
3 JOB-C
4 JOB-D
5 JOB-E
6 JOB-F
7 JOB-G
8 JOB-H
9 JOB-I
10 JOB-J
11 JOB-K
12 JOB-L
13 JOB-M
14 JOB-N
15 JOB-O
16 JOB-P
17 JOB-Q
18 JOB-R
19 JOB-S
20 JOB-T
JOB-U
JOB-V
JOB-W
JOB-X
JOB-Y
JOB-Z

That table didn't quite post the way I had hoped. Please see attachment.
Thanks,
-Dan