Get all rows where specific column value occurs more than once (filter out single occurences)
The first approach is to create an index by sessionId. But its datatype is MEDIUMTEXT...
So the approach is: normalize your data, move sessionId values into separate table and refer to it by foreign key. The reference column will be compact (4 or 8 bytes) and indexed, so its usage in the query as GROUP BY expression will improve.
The columns clientID and stage seems to be candidates to such normalization too.