ORLANDO: Good day and happiness, dear Rosalind!
JAQUES: Nay, then, God be wi' you, an you talk in blank verse.
Thus doubling can link Hamlet Senior to Claudius, if they're played by the same actor, or again the Fool to Cordelia (since they both very likely were played by Robert Armin), or Mamillius to Perdita.
So I was wondering about near-identical twins in Shakespeare -- he liked such Plautian stories as we know from The Comedy of Errors. But what about writing plays so that one actor can play both twins? Well if the twins are going to meet (and they are), you need two actors. But do you need them all the way through? I was thinking about this in Twelfth Night, and noticed this. Viola (dressed as the boy Cesario) exits Act I, Scene v after expostulating with Olivia. But the scene isn't over: Olivia broods about loving "him," then summons Malvolio to send him the ring, etc.
Next scene (II.i): enter Sebastian and Antonio. If the same actor is now playing Sebastian, the business at the end of the previous scene has given him time to change. They talk, express their love and mutual admiration, etc., and then off goes Sebastian. But Antonio stays on stage to say some more about how much he likes Sebastian and also why he (Antonio) has to be discrete. This gives the actor enough time to change again and begin the next scene (II.ii) as Viola/Cesario.
We next see Sebastian in III.iii, in a scene considerably later than Viola's last appearance in scene i; then after Sebastian's exit at the end of scene iii, Viola reappears in the next scene, but only half-way through it (again, plenty of time for the actor to change). She exits near the end of the scene, but some business after her exit between Sir Toby, Fabian, and Sir Andrew allows the actor time to change costume and re-enter as Sebastian in IV.i, where he first meets Olivia. Neither of them is in the next scene, so Sebastian doesn't have to change before he re-enters in IV.iii.
But he stays till the end of IV.iii, so Shakespeare has to write a little interchange for the beginning of the next scene (V.i) between Fabian and Feste before the actor re-enters as Viola/Cesario a few lines later. Into this last scene, of course, Sebastian will also enter, and he and Viola will reunite. But I suggest that it's only in this scene that a new actor plays Sebastian, so that he can appear on stage at the same time as Viola. (If you did a play version of The Prestige or of Dead Ringers it would be the same deal, I think.)
So I am now going to Google to see if this is generally known -- but I think it's a pretty cool thing and I am glad to have worked it out myself.
Well, apparently this is not general knowledge, since Stanley Wells generally knows. But I am sure it has to be true. So: COOL!