This does not occur in Scopus database. It is not possible to remove such duplicate citations in Google Scholar and ResearchGate database. You can message the person in RG to remove it from his profile if available in RG.
What exactly do you mean by "duplicate citation"? In case you mean that a paper has two entries in ResearchGate, see the section "Why are there duplicate research items on my profile?" in the help page https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research to get instructions how to merge two entries.