2.2.15 [XML] Section 4.2, Entity Declarations
The specification states:
The Name identifies the entity in an entity reference or, in the case of an unparsed entity, in the value of an ENTITY or ENTITIES attribute. If the same entity is declared more than once, the first declaration encountered is binding; at user option, an XML processor MAY issue a warning if entities are declared multiple times.
IE9 Mode, IE10 Mode, and IE11 Mode (All Versions)
No warning is issued if entities are declared multiple times.