I came across this in Mishna B'rura 581:27:
ואין לילך על קבר אחד ב״פ ביום אחד.
That is, roughly:
And one should not go to one grave twice in one day.
(This is in Baer Hetev :17 also, who cites the Arizal as a source.) Why not?
(I translate it as "go to" not "walk on" in light of MB 539:41, who in that way interprets the same phrase in Rama there, and who says not to walk on a grave altogether.)