Necessary to file return if filed earlier?

Not mandatory.... as per section 139(1); but always advisable to file ITR based on available income details. In case needed, it can be revised by year-end.

Sir

A person can not file his ITR1 for A.Y 2017-18 but he wants to file A.Y 2018-19, so what will be penalty for non filing of A.y. 2017-18 

Its not correlated. If at all any query or proceeding be initiated for non-filling of ITR of AY 2017-18; it will not be because of ITR for AY 2018-19 has been filed.

If it was not applicable to file ITR u/s.139.... not filed; no penalty for that.

If it is applicable for current year, of if assessee wishes to file it; he is independent to act.