It is not good to have a large similar text, as published before in another article. Most journal didn't accept that, but if you use some part of your ouw text you must mention it - you have to point where/in which journal you have that text printed. In some cases you can republish your text - for example - if you have published it bevore in a language, which is rare and the second time you publish it in English. There is quet different users and the journal will understand. You can also cite some data, received in an experiment, but you have to cite it and after that make another interpretation of that data.
You can run your documents through plagiarism check softwares such as Turnitin and Unicheck. Most publication accepts 10% similarity index excluding the references.
Free softwares are not giving accurate similarity index .If you want to publish your article in any good journal scopus/scie then use turnitin/ Ithenticate as both are best option.
The description of popular similarity checking softwares are given below.
Urkund is a web-based application that conducts server-side plagiarism detection. The solution to detect plagiarism is integrated and automated. This is a paid service that utilizes a standard email program to upload documentation and display information. This program handles 300 separate types of submissions and checks all accessible online sources. The educational sources of documents are given more priority while searching.UGC provides this software to universities of india for similarity checking.
Ithenticate for any text document is an effective tool for web-based plagiarism detection. Installation on a client computer is not necessary. Input materials are comparable with the references accessible on the site in this case. Many well-known journal publishers utilize this well-known method. It is a professional fast plagiarism checker that is easy to use. This is meant for institutional use instead of personal use, but essentially this offers a restricting feature for consumers of the specific identification of plagiarism, such as students of master and doctoral education. this allows them to check a document of up to 25,000 words.
Turnitin is a successful web-based tool. Detection and reporting is done remotely in this method. 15,000 universities and 30 Million students have now approved Turnitin because of its user-friendly design, broad repository help, robust plagiarism tests and structured report production. It can be seen as one of the best plagiarism checker.
There is no software which check plagiarism only similarity level can be checked.