As with so much of Symphogear's worldbuilding, the answer lies in the keywords.
The notion is that each Symphogear (that is, each pendant) is derived from a fragment of a whole relic that once actually existed. So there was once a supernaturally powerful spear named Gungnir, and at some point, it was broken down, perhaps by being shattered or by simply degrading over the years. Kanade (and later Hibiki) had one of those pieces, and apparently, Maria had another.
In the second season, when Maria summons the black Gungnir, everyone is surprised that there could be two Gungnir Symphogears, pointing out that Kanade's pendant was supposedly destroyed.
This is surprising, for sure. It seems unlikely that their enemy, Maria, would have a Symphogear derived from the same relic that Kanade used. But it is by no means impossible; merely unlikely. As a matter of fact, it was no coincidence that there were two Gungnir Symphogears. Why? Well...
How can Maria have an intact Gungnir pendant? Is it explained how she came to obtain such an artifact?
Remember Sakurai Ryouko? And how she was in cahoots with the Americans (specifically, the FIS)? Turns out, she smuggled fragments of Gungnir from Japan to the US. She and the FIS then developed another Gungnir-based Symphogear over there. And that's what Nastassja and co. took with them when they defected (along with Igalima, Shul Shagana, the damaged Airgetlamh, and the Shen Shou Jing; and probably also some otherwise-unusable relics to feed to the Nephilim).