In infrared spectra, the carbonyl group is usually indicated by a single strong and sharp absorption. However,

Question:

In infrared spectra, the carbonyl group is usually indicated by a single strong and sharp absorption. However, in the case of carboxylic acid anhydrides,

two peaks are observed even though the two carbonyl groups are chemically equivalent.
Explain this fact, considering what you know about the IR absorption of primary amines.