Simplify repetitive SELECT ... IN query

I have a query of the form

SELECT a, b, c from Tbl where [conditions] IN
    (SELECT a from Tbl where [conditions]
        GROUP BY a HAVING COUNT(*) > N)
    ORDER BY a DESC, b LIMIT 100

[conditions] is basically a series of AND statements and a is a column with possible duplicates. Column a + b is a unique combination. Currently I am using the GROUP BY clause simply to get the COUNT(*) > N to work. I want to return rows for column entries that satisfy the [conditions] and have at least N rows. As you can see the query is basically the same as the conditions are repeated. What is a better way to achieve the above?

Thanks.