Yes, "double hit" or "carry" in pickleball is legal under specific circumstances.
A ball hit during one continuous single direction stroke is legal, even though the ball may ben unintentionally hit twice or "carried".
"A player deliberately touching the ball with the paddle more than once" is considered a fault, according to the USAPA Rule 7.P.
However, the rule 11.A. addresses the issue of double hit directly:
Double Hits. Balls can be hit twice, but this must occur during an unintentional, continuous, single-direction stroke, by one player. If the stroke is deliberate, or not continuous or not in a single direction, or the ball is struck by a second player, it is a fault.