In the days when film cameras dominated the market, double exposure images were often created by not winding on the film between exposures. This was, of course, not always intentional, but many entertaining and intriguing images resulted, and multiple exposure photography was also used deliberately for creative effect. With digital cameras this effect is harder to achieve because once the image is captured, it is removed from the sensor, so overlaying two or more frames is difficult. However, many Canon EOS cameras now feature multiple exposure stacking of between two and nine frames.
Canon cameras including the EOS 90D, EOS 7D Mark II, EOS 6D Mark II, EOS 5D Mark IV and EOS RP have a dedicated multiple exposure function in their Shooting menu. Professional cameras such as the Canon EOS R5, EOS R6 and EOS-1D X Mark III offer additional settings. This feature is available whether you're shooting RAW or JPEG images. Within the multiple exposure shooting options, there are two methods of shooting: Function Control and Continuous Shooting. There are also several exposure settings (although not all cameras have all these options) that can be adjusted to tailor the final output to suit your needs: Additive, Average, (Comparative) Bright and (Comparative) Dark.
Bear in mind that even if your camera doesn't feature multiple exposure shooting, you can still combine images with the Compositing tool in Canon's Digital Photo Professional (DPP) software. DPP gives you even more creative options, such as adjusting the visibility and position of each image within the composite image, and selecting from a number of blending modes.