overexpressed protein sometime gives two bands

[user]

Dear all,

I'm overexpressing different proteins (phage, E. cloacae and C. freundii,
with and without his-Tag) in E. coli.
Usually I have a good expression and a single band during purification.
I wonder why I sometimes get two band, which are there all the time during
purification. They differ only very little in Mw. Probably much less than 5
kD.
I suppose, it is the same protein because it seems to bind on the column
exactly like the "one-band-protein".

Any explanations for this?

TIA,
Tina