having dual citizen

I have dual citizenship. I have expired China visa on my expired country 1 passport. Now, I want to apply for a China visa under my American passport. The travel agent is asking me not to list myself as having dual citizenship because I am applying the visa under my American passport. I don't feel this is right. If my fingerprint is taken at the entry airport in China, China will be able to tell I have previously entered China using another passport. I have nothing to hide but I think the travel agent decision is shady and not accurate. Should I list both citizenships or just list it as American?