I see it all the time at Sherbrooke on the Orange line (maybe once or twice a week), people use the exit only turnstile and turn it the opposite way and sneak in, because it’s behind where the person in the booth sits so they don’t see them doing that.
I don’t really think ticketing is the best solution, but maybe having workers (not officers) outside of the booths then people would see someone and not do it. Another solution is like @Nao1 said, having different fare gates that prevent this.
In Brussels, they have paddle doors (these one’s I don’t love, they open left and right like a sliding door instead of outwards).
Some stations have double paddle doors, you enter the area, the door closes behind you, you tap, then it opens on both sides. This prevents people from just trying to slip through with one person after another.
For busier stations, the paddle doors stay open, so when you tap you don’t have to wait for it to open. If you don’t tap and just try to walk through it closes on your face.