Need list of unique values for sumiff'ed records

Please see attached sample table if the below description isn't clear.

I have a list of records that are time block entries, (e.g. 8:00 a.m. to 9:30 a.m. = 1.5 hrs) assigned to a specific task number from 1 thru 7. So out of perhaps 50 entries I might have 7 task 1 entries, 18 task 2 entries, 4 task 3 entries, etc. The final column is the 4 digit invoice number that record is billed on. So the columns are:

col. B = time start (entry) (e.g. 8:00)
col. C = time end (entry) (e.g. 9:30)
col. D = hrs (calculated) (e.g. 1.5 hrs)
col. E = task # (entry, e.g. 7)
col. F = invoice # (entry, e.g. 4315)

My goal is to be able to do a sumif calculation for each task #1-10 totaling column 3 (total hrs) for each task. That's the easy part. But I also need to list for each task the individual invoice #s on which that task has been billed. End result being "16.5 hrs. billed for task 7 on invoices 4315, 4326 and 4410). All entries for a given task may have the same invoice #, or 4 records may have 4 different invoice # entries. So bottom line is that each task needs a collected list generated indicating the unique invoice entries (col. F) associated with any record for that task #(col. E).
Any ideas?
To make it even trickier, I would love for that list to appear in a single cell as a comma separated list (e.g. 4315,4326,4410) as opposed to a series of cells each containing one invoice #. This would be much easier to plan for and manage.
Thanks for any guidance.

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

Roger,
Once again thank you. This is perfect. I especially like the automatic column width adjustment to accommodate the string of invoice numbers as it expands. Nice touch.
One question (demonstrating my macro-ignorance here): Can the macro be set to run any time a value is changed in the "invoiced on" column on the sheet "all data entry"? The other workbook macros (or are they "just" code?) I have set up to run automatically I think, so there is no need to specifically run them. They just happen.
You've been so helpful in providing this code to us, could you also educate me just a tiny bit on how I determine or set a macro to run at specific times, or always, or on a certain event?
Hope I'm not pushing my luck here.
With great appreciation for your help with this,
Scott

Roger,
Once again thank you. This is perfect. I especially like the automatic column width adjustment to accommodate the string of invoice numbers as it expands. Nice touch.
One question (demonstrating my macro-ignorance here): Can the macro be set to run any time a value is changed in column M "Billed on MYS invoice #" on workheet "all data entry"? The other workbook macros (or are they "just" code?) I have set up to run automatically I think, so there is no need to specifically run them. They just happen.
You've been so helpful in providing this code to us, could you also educate me just a tiny bit on how I determine or set a macro to run at specific times, or always, or on a certain event?
Hope I'm not pushing my luck here.
With great appreciation for your help,
Scott

Perfect!!
Much gratitude!
Scott

p.s. And I understand events now. So thanks for the primer.