I was having the same issues as user ta007 with TUFLOW HPC resulting in the following error:
'NoXY: ERROR 2443 - Exceeded limit of more than 4 different source inflows on a single cell for HPC Solver.'
Overlapping SA inflow sources on a grid cell can be found by:
1.) Spatially joining the TUFLOW check table 'grd_check' and the SA polygon file to create a new table; AND
2.) Counting how many times each n and m cell combination occur in the new table (i.e. count how many times a grid cell is repeated in the new table).
Hope this helps.