!532 introduces an assertion to catch if multiple sources try to write to the same target. This is illegal because it would be unclear how such a situation should be resolved. This also means that providing a list to `src_grid` in config (and potentially other `src_...`) never makes sense because this spans a Cartesian product and would lead to all items in `src_...` writing to the same `tgt_...`. Example for illegal config snippet: ``` src_grid: [src_grid_1, src_grid_2] tgt_grid: tgt_grid ``` Note that the following would be a legal broadcast of a source to multiple targets: ``` src_grid: src_grid tgt_grid: [tgt_grid1, tgt_grid2] ``` I think it would be helpful to catch such configurations early and prevent users from providing lists to items where this makes no sense. This would allow us to catch such illegal configurations already when parsing the config.