“Sundry” means “various” in modern English, so strictly speaking expressions like “various and sundry” and “all and sundry” are redundant; but many redundant expressions are standard in English, as are these. “Sundry” used to mean “different from each,” which explains why the expressions weren’t redundant when they first evolved. They were a little like “each and every”: each single individual and all of them collectively.

The fact that “and sundry” now doesn’t really add anything except a rhetorical flourish to the expression may help to explain why some folks mishear this phrase as “insundry.”