For example, he infamously suggests not lying to a murderer who asks where your friend is

Actually, Kant only defended the duty not to lie out of philanthropic concerns. But if the person inquired of was actually a friend, then one might reasonably argue that you have a positive duty not to reveal his location to the murderer, since to do otherwise would be inconsistent with the implied contract between you and your friend.

To be fair, you might also have a duty to make sure that your friend is not murdered, and this might create an ethical dilemma. But ethical dilemmas are not unique to deontology.

ETA: It has also been argued that Kant's reasoning in this case was flawed since the murderer engages in a violation of a perfect duty, so the maxim of "not lying to a known murderer" is not really universalizable. But the above reasoning would go through if you replaced the murderer with someone else whom you wished to keep away from your friend out of philanthropic concerns.