How would svellsonger work if Riven already has gear on her

Short answer: it works fine — Svellsongur copies only Riven's printed text, so the gear already on her isn't copied, but she does end up with a second copy of her own abilities.

Riven's "when I attack, deal 2 to an enemy unit here for each Equipment attached to me" gets duplicated by Svellsongur, so both instances trigger on attack and each one counts all Equipment attached to her (Svellsongur included). With one other Equipment on her, that's 4 damage per trigger, 8 total, split across whatever targets you choose for each trigger. Whatever the pre-existing gear grants her doesn't feed into the copy — copies take the object's original traits regardless of later alterations, and attachment order doesn't matter (FAQ #8410; Effect Text only becomes active while Attached, Rule 724). Stacking more Svellsongurs gives one extra instance each, not doubled instances — three Svellsongurs on Riven means four total instances of her text, not eight.

One caveat: this only matters for her text. Attached Equipment still gives its normal Might bonus on top; Svellsongur has no Might bonus of its own and doesn't amplify it.