Assuming that you are 100% going to law school after your undergraduate degree, I offer the following advice:
You only get that 4 years to do an undergrad once so you should pick you major carefully. I would agree that doing an undergrad in law will certainly help you in law school, as you will already be familiar with some terms and general principles in the field prior to entering law school. This may be a slight edge over your colleges who are coming from a non-legal field and are completely unfamiliar with these concepts. However, this advantage will be small, as the difference in depth of legal content that you will be exposed to in law school will not be comparable to your undergraduate degree. The alternative would be to do a degree in an unrelated field which won't necessarily help you immediately in law school, but may pay dividends later on. For example having a science or engineering background will undoubtedly make you more attractive to IP law firms or having a finance background may give you an advantage in certain aspects of corporate law. However, having a law undergraduate background will not give you any sort of advantage in terms of employment opportunities because everyone has a law degree (unless your grades are better as a result of doing law as for your undergrad) and therefore having an undergrad in law doesn't mean much. Therefore it's really a cost-benefit analysis between whether you want the short-term head start on your law school peers by already being familiar with law (which may lead to you getting slightly better grades and therefore better job prospects) or if you want to diversify your skill set by having knowledge that may come in handy later on that may make you especially valuable to employers (or may have no effect, depending on what practice you want to go into).