How to probe twice while mesh bed leveling with a BLTouch setup
For the 1.1.x (1.1.9) bugfix firmware version, the multiple probing is defined in the configuration.h file.
// The number of probes to perform at each point.
//   Set to 2 for a fast/slow probe, using the second probe result.
//   Set to 3 or more for slow probes, averaging the results.
//#define MULTIPLE_PROBING 2

You should change the configuration to include the multiple probing by uncommenting the line:
#define MULTIPLE_PROBING 2


Note that there is question: &quot;Updating Marlin Firmware - Step by Step Guide&quot; that shows how you should modify the sources.
How to compile and upload/install the compiled binary on your printer controller board depends on the controller board version and type.