Dear Ms. Fallahi 

You can use some software such as : plagiarism detector ,  plagiarisma , or write check

Please use this link : https://drive.google.com/a/usmba.ac.ma/file/d/0ByNPgOEzijZRSFBWWVZiQlU3akk/[email protected]&ts=57c52b44&actionButton=1

@Saad Motahhir  : I can't open it.It says it needs permission!

@Saad Motahhir. Thank you very much for the file.

What percentage of duplicate is accepted in journals?

Welcome, i think lower than 20%

Is there a free, accurate plagiarism checker?

You can also explore the following RG links:

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

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

You can check oxphrase, an accurate plagiarism checker, offering a free trial.

https://www.oxphrase.com