Hi,
Whether MSM does a single or double probe operation is controlled by the "Probe Slow Feed Rate" parameters in the "Probing Op Parameters" panel. The Probing parameters are documented in section 8.5 of the user manual and section 8.5.1.5 covers the Probe Slow FR (Feed Rate) parameter.
If the slow FR is = 0, MSM does a single probe action, if the feed rate is non-zero, then a double probe is done.