Add warning if a variable name is repeated twice in match.on argument

[nielsenrich]

Roozbeh included the same variable twice in a long list of matching variables and got this unhelpful error message:
Calculating Mahalanobis distances...
Error in solve.default(cov(dataset[, matchVars])) :
Lapack routine dgesv: system is exactly singular: U[18,18] = 0