Check if transaction already exists during transaction validation

[samuelmanzanera]

Description

Trying to send the same transaction from the node should be prevented during transaction validation by checking the existence of the transaction

Fixes #678

Type of change

• Enhancement

How Has This Been Tested?

If you send an existing transaction from the node an error should be triggered.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules