Data must have 1 PO Number per 1 Vendor

[WascoWarrior]

Not sure how to check for this?

In the above example, row 5 is invalid because that vendor number was already used in the previous PO.
Row 7 is invalid because that PO was already used with vendor # 5216.
Was thinking of using another field with an OFFSET but not sure how to code.
Suggestions?

 

[iggydarsa]

Assuming:
Your table on the screenshot starts on column A
and PONumbers are sorted then

you can use this formula on H2 (or any empty column starting on row 2)

=IF(OR(INDEX(A:A,MATCH(B2, B:B, 0))<>A2, VLOOKUP(A2, A:B, 2, FALSE)<>B2), "Invalid", "")

EDIT:
I see your data starts on Row 3, so paste this formula on Row 3 of any available empty column:

=IF(OR(INDEX(A:A,MATCH(B3, B:B, 0))<>A3, VLOOKUP(A3, A:B, 2, FALSE)<>B3), "Invalid", "")

 

[Cubist]

Another option:

=IF(ROWS(UNIQUE(FILTER($A$3:A3,$B$3:B3=B3)))>1,"Invalid","Valid")

 

[WascoWarrior]

Assuming:
Your table on the screenshot starts on column A
and PONumbers are sorted then

you can use this formula on H2 (or any empty column starting on row 2)

=IF(OR(INDEX(A:A,MATCH(B2, B:B, 0))<>A2, VLOOKUP(A2, A:B, 2, FALSE)<>B2), "Invalid", "")

EDIT:
I see your data starts on Row 3, so paste this formula on Row 3 of any available empty column:

=IF(OR(INDEX(A:A,MATCH(B3, B:B, 0))<>A3, VLOOKUP(A3, A:B, 2, FALSE)<>B3), "Invalid", "")

 

[WascoWarrior]

Code works great! I wrapped it around an IFERROR so I don't get #N/A on a failed lookup as I copied the formula down the column.
Also modified the code since the PO started in column B.
=IFERROR(IF(OR(INDEX(B:B,MATCH(C3, C:C, 0))<>B3, VLOOKUP(B3, B:C, 2, FALSE)<>C3), "Invalid PO/Vendor", ""),"")