Why are there two identical photos taken by my Canon SLR camera in the memory card?

This is because files are stored in RAW and JPEG formats.

SLR cameras store photos in two formats. One is the RAW file used for post-adjustment when saving more original information (the suffix of Canon RAW file is generally CR2, and the suffix of Nikon is NEF), and the other is the JPEG file which is generally more open and suitable for direct drawing. Almost all drawing software can open JPEG files.

JPEG is compressed. Although the file is relatively small, it will lose some picture quality details. The format of the original file is much larger and varies from manufacturer to manufacturer. Many softwares can't be opened or displayed incorrectly. Both have their advantages and disadvantages. Generally choose the storage format according to your own needs.

If Canon camera doesn't want to store two formats, you can choose the format to store according to the following operations:

In the menu, you can choose the storage format: RAW format is stored separately, JPEG format is stored separately (three JPEG images are optional), and both RAW and JPEG are stored.