Is there an official symbol for repetition in a table
The ditto mark, (also known as double quotation marks) would seem appropriate
The ditto mark (〃)1 is a typographic symbol indicating that the word(s) or figure(s) above it are to be repeated.
Wikipedia
I would like to add that, apart from the "Ditto" and ditto mark ("), "Do" is also used to signify that the value of a particular field is same as that of the preceding field.
"Do" perhaps refers to abbreviated form of "Ditto".