Is another person proposing to a girl, after one has performed Salatul-Istikhara regarding marrying her, considered among the signs of Istikhara?

After performing Istikhara, one should proceed with the intended matter and not abandon it unless Allah diverts them from it. If the other suitor is accepted and you are rejected, perhaps Allah has diverted you from her out of mercy for you, especially if you performed Istikhara well and put your trust in Allah. And if the new suitor is rejected and you are accepted, then that might be from Allah's grace and an answer to your supplication.