This is an error caused by two accounts having the same hard-coded MatchStrings on the Accounts tab. In the example below, accounts 4050 and 4060, both have the matchstrings "4050"
To resolve this, find a cell in colJ that has a formula that includes the function Left(). Then copy from that cell to the row with the duplicate Matchcells. The formula will look something like this: =LEFT(D165,FIND(" · ", D165)-1)