Aggregating multiple encoded categorical values
If individual categories are important in your analysis, you could split the category column into multiple columns based on the amount of different category values then pivot your data set to have multiple row entries per category. 

Visually

recordid | category | value
-------- | -------- | -----
   1     |   a      |  5
   2     |   a,b,c  |  10


would become

recordid | category |  value
-------- | -------- | ------
   1     |   a      |   5
   2     |   a      |   10
   2     |   b      |   10
   2     |   c      |   10


You could then make further aggregations or transformations as necessary.

Otherwise you could consider "a" and "abc" and "abcd" a new category, i.e. an item with categories "abc" is different from an item with just the category "a"