Whenever you are greeted with a greeting, then you greet them back better than it or at least return it; indeed Lord Allah is a Reckoner upon everything.
The believers are commanded through verse 24: 61: "When you enter houses, give greetings of peace upon yourselves; a greeting from Lord, blessed and wholesome".
It is told in verse 58: 8 that the hypocrites during Messenger's time politely greet with words even Lord doesn’t greet him with, and will think in mind that why didn't Lord punish them for their crooked plans. Whenever the Jews and hypocrites meet the Messenger, they greeted him: "Let destruction be upon you" by twisting their tongues. So Messenger greeted back to them: "Upon you!", and if he realized that greeting is normal, he used: "Upon you too".
As there is no group of believers anywhere in the world, the odd believer whenever is greeted by the other people, will greet back it in a better way or at least in a fairway. But any among the Arabic Quran reading Fujjar upon whom Lord’s wrath and curse have happened if greets the believer, the believer should only return in the way how he is greeted. Since the believer is commanded to do Jihad with Adhikr against the Fujjar, he will not greet them first. But he will consider mankind alike and will not show any discrimination based on religion, sex, color, etc. If the Fujjar are greeted by a person from other people, they will not greet him back.
As Lord has greeted back for the Messenger's greeting, the believer should greet back for the greeting. See explanation 2: 62, 165-167; 4: 63 and 17: 1.