Based on MSCD, I gather that you’d say that parties enter into an agreement, rather than simply enter it. (See, for example, MSCD 2.21 and 8.18.) The former usage is certainly common and, just as certainly, redundant. Why not use just enter?
Prepositions have a way of glomming on to verbs, turning them into prepositional (or “two-word”) verbs, even when it seems that the verbs were doing just fine without the preposition. This is something my daughter and I trade notes about. Some examples using up:
“Clean up your room!,” shrieked Susan’s mother.
“Rest up. We’ll head back at sundown,” said Sergeant Jennings.
“Wait up!,” cried Jorge despairingly.
In each of these examples the up is, to varying degrees, extraneous.
Currently, my favorite redundant preposition is the on in to hate on, as in “Stop Hating on NAFTA” (the title of a Washington Post op-ed piece).
So I’m sympathetic to the notion that the into in enter into a contract might be superfluous. But English is full of legitimate two-word verbs. (Click here for an entire dictionary’s worth of them.) And it would never have crossed my mind to say “Acme and Widgetco entered a merger agreement.”
Tom’s concern is that because to enter means “to go into,” it would be pointless to follow enter with into. But it’s best not to be too literal-minded when dealing with two-word verbs. Consider, for example, to turn up, meaning “to arrive unexpectedly,” as in “He turned up at my house on Tuesday morning.” I defy you to arrive at that meaning by combining the respective meanings of to turn and up.
I could be swayed by popular usage, but Google offered me 143,000 hits for “entered a contract” and 1,260,000 hits for “entered into a contract.
So I’m sticking with enter into. But I invite you, dear reader, to cast your vote in the poll below.