I'm not aware that anyone has ever been shown to have an allergy per se to MDMA. Excessive idiosyncratic reactions, certainly, but not in the true sense of the word allergy.
One pill could kill you, sure.
Contaminants (PMA, DXM etc) are one issue. What you do on the pill is another, and perhaps the most likely one: hyperthermia, hyponatremia, etc. You might do something silly and injure yourself fatally. You might have an interaction with another medication (MAOIs for example) that could prove lethal. You could use it with other drugs, and not handle the combination and die.