Burp Repeater
The repeater is very useful when there is a need to send a usually complex request multiple times with little changes in-between.
Some usual use cases are when trying injections manually, such as SQL injections, or command injections.
Once a request is caught, send it to the repeater. You can use CTRL+R.
Notes
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You have one tab per request.
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Edit the request, URL encode it if needed (CTRL+U), and send it
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There is an inspector on the right, for those that do not want to edit the request manually