  The study of the chemistry of the stellar populations in Globular Clusters
(GCs) is a fundamental task to unveil their formation in the high-redshift
universe and to reconstruct the build up of our Galaxy. Using metallicity
estimates from BP/RP low-resolution Gaia DR3 spectra, a recent work presented
the surprising detection of two stellar populations with distinct metallicities
in the stellar stream of the GC NGC 5904, otherwise considered a mono-metallic
system. The presence of these two populations, with [Fe/H]=-1.4 and [Fe/H]=-2.0
dex, was taken as the evidence of a merger origin of the cluster. In this
Letter, using the same data set complemented by new robust metallicity
estimates, we carry out a detailed analysis of the metallicity distribution of
stars belonging both to the cluster and to its stellar stream, explicitly
focusing on the subtle effects of data systematics. We demonstrate that the
population at [Fe/H]~-2.0 dex is a data artefact due to error systematics,
affecting especially faint stars. The new higher quality metallicity sample
corroborates this finding, and it indicates the presence of only one population
of stars with metallicity of [Fe/H]~-1.3 dex, in agreement with previous
literature studies. We, therefore, conclude that both NGC 5904 and its stellar
stream are mono-metallic systems, and emphasize the need of carefully examining
systematic effects in large and complex data.
