return average counts for distinct samples but with same name

I need a formula that would allow me to return the average count value (column E) for unique samples (column D) from total counts (column A) from a list of samples (column B). Some samples have the same name (e.g. A), but is considered unique because they are located between rows of unique samples (e.g. 1a and 2a). I can get the average by calculating samples A, 1a, A, and 2a one at a time, but is tedious and would like a formula that can do an entire column all at once to return the average number for "unique" samples (like in column E). i've attached a spreadsheet to show you what I mean.

Thanks for any advice.