Fusions can be created from any two silver doppelgangers, consuming them in the process.
Many users collect doppelgangers, and there are Trophies to signify how many you've obtained.
Legacy Doppelgangers Edit
Legacy Doppelgangers are doppelgangers that were created before the fusion script was rewritten to create fusions from doppels instead of customs. Legacy Doppelgangers cannot be fused.