Removing repeated duplicated characters

try this

SELECT @Parameter AS 'BEFORE'
BEGIN
WHILE CHARINDEX(',,', @Parameter) > 0
    BEGIN
        SELECT @Parameter = REPLACE(@Parameter, ',,',',') 
    END
SELECT @Parameter AS 'AFTER'
END

I would suggest a UDF to do this. Since the UDF I am about to suggest doesn't touch any tables, the performance should be pretty good.

CREATE Function [dbo].[CleanDuplicates](@Data VarChar(8000), @DuplicateChar VarChar(1))
Returns VarChar(8000)
WITH SCHEMABINDING
AS
Begin

    Set @Data = @DuplicateChar + @Data

    While PATINDEX('%' + @DuplicateChar + @DuplicateChar + '%',@Data) > 0
        Set @Data = REPLACE(@Data, @DuplicateChar + @DuplicateChar,@DuplicateChar)

    Return Right(@Data, Len(@Data)-1)

End

You can test the function like this:

Select dbo.CleanDuplicates(',,,', ',')
Select dbo.CleanDuplicates(',,,sam,,bob,', ',')

This works for strings that are exclusively commas or have up to 398 contiguous commas.

 SELECT 
     CASE 
         WHEN TargetString NOT LIKE '%[^,]%' 
             THEN '' /*The string is exclusively commas*/
         ELSE 
            REPLACE(REPLACE(REPLACE(REPLACE(REPLACE(TargetString,
            REPLICATE(',',16),','), /*399/16 = 24 remainder 15*/
            REPLICATE(',',8),','),  /* 39/ 8 =  4 remainder 7*/
            REPLICATE(',',4),','),  /* 11/ 4 =  2 remainder 3*/
            REPLICATE(',',2),','),  /*  5/ 2 =  2 remainder 1*/
            REPLICATE(',',2),',')   /*  3/ 2 =  1 remainder 1*/
         END
 FROM T    

Add extra powers of 2 at the top if you need more or remove from the top if you need less. The comments by each stage indicate the smallest number that this stage will not deal with successfully.

All the comment lines are in this format

/*  L/D    =  Q remainder R */

D:    Corresponds to the length of the string generated by `REPLICATE`
R:    Is always D-1
Q+R:  Form L for the next step

So to extend the series upwards with another REPLICATE(',',32),',') stage

D = 32 
R = 31
Q = 368 (399-31)
L = (368 * 32) + 31 = 11807

So that would deal with sections of commas up to 11,806 characters.