No, you can only appear for NEET-PG only once during the testing window.
It will be deemed a fraudulent activity if one appears more than once. Admit Cards and photo IDs of the candidates will be checked, and digital images and fingerprints will be captured at the test centre.
The NEET-PG 2017 Demo test is a sample of the computer-based exam that is intended to familiarise candidates with the navigation and functionality of the exam.
It includes a tutorial on using the various features of the exam like moving between screens, selecting, de-selecting, marking and reviewing responses etc., as well as some sample questions which will allow candidates to try out these features.
Note that these sample questions are not representative of the content or difficulty level of the actual exam.