It's very common to come across error coins, and even ordinary coins,
that appear to have a "double rim". A true double rim is, of course,
impossible, as there is only a single rim gutter in the die face.
There is more than one cause for this effect:
1. Finning. When striking pressure is a bit too high, or one of the
dies is slightly tilted, metal is forced up into the gap between the
die neck and the working face of the collar. This is called a "fin"
or a "finned rim". Anyway, the fin lies outside the true rim and, if
it is low, appears to constitute an extra rim.
2. A false rim is sometimes seen in misaligned dies and minor,
uncentered broadstrikes. Coin metal rises into the rim gutter,
marking the location of the true rim, but a false rim is created
lateral to that as metal is pushed against the collar (in the case of
an in-collar misalignment), or it simply bulges out lateral to the
true rim line (in the case of an uncentered broadstrike).
3. In the case of a misalignment, or a coin struck in an oversized
collar (rare), you sometimes see a pressure ridge internal to what
looks like the true rim. Neither is a rim in the truest sense. Both
are pressure ridges created by the die being too far from the working
face of the collar. The inside pressure ridge marks the location of
the rim gutter (although it hasn't filled it), and the outside
pressure ridge marks where metal has piled up against the working
face of the collar.
4. Die fatigue. Sometimes in late die states, the working face of
the die deforms in such a way as to create a false rim internal to
the true rim. This is best seen in copper-plated zinc cents.