Emor el hakohanim bnei Aharon v'amarta aleihem... There are two redundancies here: 1) the double emor... v'amarta 2) Kohanim = bnei Aharon, so why say both?
The Kohanim are about to get mitzvos that place more restrictions on them than on an ordinary person. The Torah therefore prefaces the commandments with an introduction: Emor el hakohanim bnei Aharon, tell the Kohanim that they are the decendents of the great Aharon hakohen, they are the heirs to his legacy. You have more restrictions not to make your life difficult, but because you are holy and special. (Chasam Sofer)
Chazal learn from the double emor... v'amarta the idea "l'hazhir gedolim al ha'ketanim." I would say this goes hand in hand with the Chasam Sofer. The way to teach a child is to first make him feel special, make him feel privileged to be given mitzvos. That is the preface needed before anything else can be taught.
(Lzecher nishmas Eliezer been Meir)