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.

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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.

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In §7:

We now prove the important result announced in §3:

7.x Lemma. Every regular doodad is a widgit.

Proof. Proof goes here.