autofill with adjacent cell reference

I have a large set of data in column B with multiple duplicates and I am in the middle of entering data into column A.

I would like to find a way that once I enter the data into column A all other "A" fields adjacent to the same B fields will auto populate.

I do not have a data set to reference for column A as I am finding the data in another software and cannot export it to excel.


Example:
Colum A Column B

ABC	123858
	555658
	123585

I would like that column A3 auto fill to abc because B1 and B3 are the exact same.

May be this......

In A3

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This would require VBA.
Right click on the sheet tab and "View Code" Then paste this in (assumes you are typing into Column A and looking in B for matches)

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Glad to help