See the following discussions for recommendations:

https://www.researchgate.net/post/Best_Free_Plagarism_software_for_research_papers

https://www.researchgate.net/post/What-is-the-most-powerful-free-plagiarism-checker-software

https://www.researchgate.net/post/Can_anyone_suggest_free_Plagiarism_softwares

https://www.researchgate.net/post/Looking_for_free_Plagiarism_Detection_software_or_site

https://www.researchgate.net/post/What-software-do-you-recommend-for-plagiarism-checking

https://www.researchgate.net/post/Best_plagiarism_checker_software_for_research_papers

https://www.researchgate.net/post/Dear-researchers-Please-i-need-a-plagiarism-checker-Thank-you-so-much

https://www.researchgate.net/post/Did-you-have-a-good-plagiarism-checker-like-that-used-in-an-impacted-and-indexed-journals

https://www.researchgate.net/post/Which-is-the-best-plagiarism-checker-free-software-or-webpage-for-student

https://www.researchgate.net/post/What_is_the_best_free_plagiarism_checker_to_use_as_a_student

https://www.researchgate.net/post/As-a-researcher-do-you-think-that-similarity-check-or-plagiarism-check-soft-wares-are-fair-in-all-cases-What-is-your-experience

https://www.researchgate.net/post/Respected_researchers_Can_anyone_recommend_a_free_plagiarism_checker_that_will_assist_me_in_knowing_my_errors_and_help_me_targeting_good_journals

Thank you, Wolfgang R. Dick . Many interesting points in this discussions.