Why is "bis" used when saying nid(ethylenediamine)?

For coordination compounds, prefixes are used to designate the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-. However, if the ligand already contains a prefix (e.g. ethylenediamine) or if it contains polydentate ligands which can attach at more than one binding site, the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead. So bis means there are two en ligands.