From my long experience in helping Contact Form 7 users on the support forum I can say that the most common reason behind troubles is interference from other plugins, or with the theme which the user uses.

Receiving multiple emails with the same content from the same sender in a short period of time — I often receive support requests like this.

Although some suppose these are caused by a bug or made by spam bots, in most cases they are created by real humans. It is real well meaning people who are making multiple submissions through contact forms on their sites.

They ask me if they can make Contact Form 7 prevent such multiple submissions. Actually this is not difficult at all. You have some options to do so, such as disabling the Submit button after the first submission.

You might think that such functions should be activated by default if it is not difficult. In reality such functions are intentionally not implemented in this plugin — because it obviously leads to a terrible outcome.