Noah Smith writes this post on the advantages and disadvantages of coming to an econ graduate program from a physics undergraduate major. The upshot: It helps a little and it hurts a little. The same can be said for coming to econ from econ.
The big advantage doing econ from undergrad all the way through is that when you get to grad school you already speak the language and know the intuition - at least on the micro side of things.
There are two potential disadvantages to coming to this path. First, if you're like me, your undergraduate focused on policy economics, and you learned about things like IS-LM, fiscal policy, and monetary policy on the macro side; and public goods, externalities, and other market failures on the micro side. This knowledge will not help you survive the core micro or macro sequence in grad school. In fact, it will likely hurt you (especially in the macro side).
The other disadvantage is not enough math. If you're like me you took a few semesters of calculus, some stats, an econometrics course, and maybe even a decent math econ class and thought you were hot shit. What do you win for all that? A rude effing awakening when you get to grad school. If you do econ, pair it with a math and/or stats minor, at least - maybe even a double major.