This error message is displayed when the first column of the dataview has duplicated values in, e.g when there is a toolkit or SQL-toolkit sampler which returns several rows with same value in the first column.
There are 2 possible solutions
1. Disabled the error through Gateway Setup Editor (GSE)
GSE > Operating environment > Advanced tab > Duplicate row alerts > None
2. Remove duplicates.
Ensure that the first column of the dataview does not contain duplicates.