Why does Creon give Antigone a chance to claim she wasn't aware of his proclamation? Does Creon have some doubts about the justice of his action?
When Antigone is brought before Creon to account for her crime of burying her brother contrary to Creon's decree, Creon asks her: "Did you know it had been announced not to do this?"
As you suggest, Creon may have had some lingering doubts about the correctness of his decree. In fact, near the end of the play, his doubt is shown because he is persuaded by the seer Teresias, and by his own royal council to free Antigone from her prison. This is partly because Creon fears the punishments that Teresias prophesies for him; it is also, however, because Creon begins to see the correctness of Antigone's position that one must always obey the divine requirement to bury the dead. As Creon says:
I bound her and I will go and release her,
for I fear that it is best to keep
the established laws as long as one lives.