What is most likely the reason Lincoln repeats his declaration that "all persons held as slaves . . . shall be . . . free" in his Emancipation Proclamation? because time has passed since Lincoln’s initial proposal to emancipate slaves, he must differentiate between the two proposals

because Lincoln wishes to test what he assumes is the rhetorical power of repetition when addressing the nation’s citizens, Lincoln uses it effectively in this declaration

because he knows that the subject is certain to provoke strong public opinion, Lincoln chooses to emphasize the struggle that will result from emancipation of slaves

because Lincoln first offered a deadline for pro-slavery states to return to Union control, he repeats the main points of that order