How to Count Unique Values From a Repeated List in Priority with a Cell - Google Spreadsheets

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I have a spreadsheet with four columns:

question_id  user_id   unique_question_ids   # of unique_user_ids
 X            11        X                     ? (=2)
 X            12        Y                     ? (=3)
 X            12
 X            12
 Y            13
 Y            14
 Y            15

The first two columns are questions and their corresponding users and include repeats of both.

The objective is this: I want to count the number of unique users for each unique question.

I started with first finding the unique_questions which I found using the UNIQUE function. This listed what questions are unique in the unique_question_ids column (i.e. X, Y)

Now I want to count the number of unique users that each unique question has? The other problem is that I do not know where X and Y start, however they are still sorted in order (real spreadsheet is very large).

How would I go about doing this? I am thinking I could use COUNTIFS but this doesn't count for unique values. I also thinking of using a function that would return the range of where X or Y is located in the question_id column, and then count in the next column (i.e. user_id) for unique values. But I cannot find a function that returns the cell range of a value in a column. I am also doing this on Google Spreadsheets.

Any thoughts or ideas would be appreciated, thanks