HP LaserJet P4010 and P4510 Series - Software Technical Reference
Page 176
...single-sided pages of settings is known as print mode. If a duplexing unit is installed and manual duplexing is enabled in the driver, the printer automatically prints on both sides of media types that are supported for automatic duplexing (Letter, A4, Legal, and 8.5x13 only), and... available in the document options, select the Allow Manual Duplexing option on the second side of the media because the HP LaserJet P4014, P4014n, P4015n, P4015tn, P4515n and P4515tn printer models do not have a duplexing unit installed. A different print mode is specified in the print job. Manually print...
...single-sided pages of settings is known as print mode. If a duplexing unit is installed and manual duplexing is enabled in the driver, the printer automatically prints on both sides of media types that are supported for automatic duplexing (Letter, A4, Legal, and 8.5x13 only), and... available in the document options, select the Allow Manual Duplexing option on the second side of the media because the HP LaserJet P4014, P4014n, P4015n, P4015tn, P4515n and P4515tn printer models do not have a duplexing unit installed. A different print mode is specified in the print job. Manually print...