I would give only one of them a number. The other one would not be numbered at all, but would refer to the numbered version.
Depending on the text at hand, I would thus choose one of:
In §3:
A very important result is:
3.x Lemma. Every regular doodad is a widgit.
In §7:
We recall Lemma 3.x:
Lemma. Every regular doodad is a widgit.
Proof. Proof goes here.
or
In §3:
A very important result is:
Lemma. Every regular doodad is a widgit.
In §7:
We now prove the important result announced in §3:
7.x Lemma. Every regular doodad is a widgit.
Proof. Proof goes here.