Need list of unique values for sumiff'ed records

Roger,
That is great code, and it works perfectly in my sample sheet, so thank you very, very much. I thought I would be smart enough to apply whatever solution someone came up with to our real time tracking spreadsheet (attached), but once I read through your lines I began questioning my ability to accurately transpose it to our somewhat more complex sheet. The questions stem from not really being able to discern what were references to cells and what wasn't (I'm obviously not a programmer). Will the code you supplied work with the attached work book? The data entry is on the first sheet ALL DATA ENTRY with invoice #s in column M, task #s in column K, with the summary on the SUMMARY sheet. I've created the destination column (column L) for the invoice #s to be collected to on the sheet SUMMARY.
Doable?
With thanks for any further help (and great gratitude for the help so far),
Scott