Thank you for the prompt reply. My impression is that the problem I had was different then adding the same prefab twice: each prefab was added once. My problem was that I was not aware of PoolBoss adding the component "PoolableInfo" to the prefab. So when I duplicated a prefab and then I changed it from its original version, the copy still contained "PoolableInfo" (with the name of the first prefab). I did unpack all prefabs, changed their names and repacked them, but that was not useful because the PoolableInfo was still there, with the same one old name in two different prefabs.
Anyway, all solved now, thank you for the clarifications and for the prompt support.