Using your posted workbook...here are two approaches:
1) A regular formula (not an array formula) that returns just Col_A matches
2) A regular formula (not an array formula) that returns concatenated Col_A/Col_B matches
One other thing...
If you convert the lookup range to an Excel Table (see the attached file):
1) you can reference the table fields directly
2) the table will automatically expand and contract to accommodate the data
3) you won't need to make Excel go through the work of calculating on 70,000 rows
So the concatenated field formula would be:
Yes, it's more verbose, but it's also very descriptive.
Does that help?
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