Using doubles can’t be overstated, though. A TV movie about the Dionne quintuplets was made about 15 years ago with no trick photography at all.
Since there were no living identical quints in the world at the time (in fact, I think the Dionnes are the only ones definitively known), and not even an identical set of quads the right age and gender to play the Dionnes, the casting director just put out a call for identical twins and triplets who fit the general description of the Dionnes. She lucked out, and found two sets of triplet girls who looked very similar to one another-- that is, each set was identical, and one set strongly resembled the other. They were about 8 months apart, so one set averaged taller, but the Dionnes were never all the same height, anyway.
In long shots, they used one set, and two girls from the other set, and in full face shots, used only the older set-- only the older set ever had lines.
I said there was not trick photography, and there wasn’t, in the editing booth, nor any computer manipulation, but sometimes the camera would pan in and out, and when it panned in, one girl might speak a line, while the others rearranged themselves, so the next one to deliver a line was standing in the correct place, in order to have always one from the older set speaking. This could happen in a scene where each of the five had a line, but only the older three actresses delivered them.
It was very effective. From behind, or from a distance, and dressed alike, they did look like a matched set of five.
Doubles are used all the time in films to save time and money, or bruises on a lead actor.
If a script calls for a remote location shot, and it would be necessary to lose a week in the studio while the lead actor flew out to whatever the location is, got over jet lag, filmed for a couple of days, flew back, got over jet lag, and then it was Sunday, so filming didn’t resume until Monday.
If the types of things done on location don’t require the actors’ face to be shown a lot interacting with the location scenery, it’s easier to send a double to do the location filming (2 to 3 hours of footage may be needed in order to edit down to the 15 minutes needed for the movie, and that could take two days with setting up and tearing down equipment, setting light for each shot, etc.), and also take several shots that can be greenscreened into the footage with a few 2 or 3 minute close-ups of the actor’s face.
Then, of course, there are stunt doubles.
Actor’s aren’t chickens, and they aren’t out of shape, but sometimes stunts are very specialized; sometimes they require several takes to get right, and stunt people are cheaper to pay to film a scene over and over; and stunt people are always under the risk of cuts and bruises. You don’t need to be constantly hiding these on your lead actor.
Then there are body doubles. If an actor in under 18, a body double has to be used for a sex scene, and some actors over 18 insist on them for sex scenes anyway. An actor or actress recovering from surgery may have a scar they don’t want filmed. An actress could be pregnant, and not showing under her clothes, be be obvious, when naked for a sex scene, or various other reasons.
So considering how often doubles are used, Hollywood doesn’t bat an eye at using them for people playing their own twin.