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.