@Aldebera To be honest, any average player can tell who's the real Mirage. If you try to ult while escaping those kinds of players, you're going to get knocked if you did it right in front of them. The real Mirage's arms glow when ulting and any player who knows that will shoot the real Mirage everytime.
Also, just to let you know, when an Alter goes through their portal, if they have the perk that keeps players highlighted for a bit longer, (showing health bars and a glow on portal exit), the real Mirage will be the only one visibly marked. I don't think Respawn thought that one through.
If you had died to an Alter who phased into your building, this is how they knew which one you were. (But yes, wall hackers are a major issue in the game.) Although, even without the perk, enemy players glow while in phase, be it an ability from Wraith, Ash, or Alter, so if players pay enough attention, you're going to get knocked down as the decoys don't seem to glow.