Lookup to return the lowest time.

I have a number of case numbers that either have open/closed status, and several cases may be opened and closed multiple times - still the same number. I will then try the earliest time of the first time the case is closed.
I have counted on duplicates and said that if there is more than 1 closed on the case, the lowest value should be returned, but PBI returns the lowest value for the entire column.

Is there anybody who can help?

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