Calculate average of time difference based on non-duplicate criteria

Hello,

Can someone please help me come up with a formula to take the average of the difference between times in column C (C2:C16) and column B (B2:B16), but not include any duplicate times based on trades in column A (A2:A16)?

Basically I am trying to calculate the average time a trade is held for, in one cell, C18. Column A is trade ID's with different trade $ amounts (not shown) which is why there's duplicate ID numbers. I would like to only find the difference between times based on each Trade ID once. Then I'd like to take the average of the entire difference of those times.

The answer I'm looking for is in F2, but that was manually calculated. My attempt at a formula is in H2 but that's incorrect, you can ignore the rest of the spreadsheet. Please let me know if this is possible. Any help or guidance is much appreciated! Sample spreadsheet attached.

Try this:=SUMPRODUCT((C2:C16-B2:B16)*(A2:A16<>A3:A17))/SUM(1/COUNTIF(A2:A16,A2:A16))

Thank you alansidman, josephteh and kvsrinivasamurthy! I appreciate the quick response and help. Those formulas worked perfectly. Unfortunately I wasn't able to use the Power Query as I could not find the option to paste the code in a Blank Query on my version of Excel. Maybe because it's a Mac version?

Nonetheless, the problem is solved. Thanks again!