Set repeating IDs till first record repeats (bulk load csv file)

I have a file that I imported via bulk-insert and I want to assign group IDs/sequences. I would like to assign the IDs till the first record with the first character is repeated. In this example its "A".

The challenge I have is how to achieve this example and set the IDs like this example:

Here is one way to do it, but given the limited info you provided I will make the following assumptions:

**The data in your table has some order to it. This obviously will not work if that is not the case. I used an ID, you use what you have.

**The first row in the table has the character you are looking for.

CREATE TABLE #tmp(ID int,   [data] varchar(20))
INSERT INTO #tmp
VALUES
(1, 'A000abcefd'),
(2, 'E00asoaskdaok'),
(3, 'C000dasdasok'),
(4, 'A100abcasds'),
(5, 'E100aandas'),
(6, 'C100adsokdas')


DECLARE @CHAR varchar(1)

SELECT @CHAR = (SELECT TOP 1 SUBSTRING([data],1,1) FROM #tmp ORDER BY ID)

SELECT SUM(CASE WHEN SUBSTRING([data],1,1) = @CHAR THEN 1 ELSE 0 END) 
                    OVER(ORDER BY ID ROWS BETWEEN UNBOUNDED PRECEDING and CURRENT ROW) SeqNum
,[data]
FROM #tmp