[Jake Starr]

Should I be worried ?

No. Several here on this forum have experienced the same thing. I shoot USPSA and on mor than one occasion I have bump fired a "double-tap."

Is this to be considered as the result of a state of mind (stress) and how to avoid this in the future (during rapid firing) ? Maybe I should work on my manual of arms or shooting technique ?

Stress or excitement, either or but the only way to overcome this is to practice, practice and more practice....remember in the shooting sports, "slow is smooth and smooth is fast."