I recently finished a MSc and I was looking for a PhD. I noticed that some positions offer a PhD only through a MRes of 1-2 years, for a total of 4 years.
Does it make sense applying for another master or is it just redundant?
The MRes degree teaches you the basics of how to conduct research and use research methods according to your chosen field. Depending on the field you select, you use different types of research methods and you get a chance to put them in practice. If you choose to do the MRes degree, it will be considered a higher degree than a taught or research based MSc degree, but lower than the MPhil. The MRes would be seen as a good foundation for you to progress to a PhD and produce original output using the research methods that you have learned during your MRes degree.
As an example, in Computer Science, you only need a 1st class honours bachelors to get into the PhD route and you learn which research method you will use whilst on the PhD route.
You are more likely to be able to bypass the MSc if you have a good first degree. Where I did my Ph.D (University College London), it was a requirement that you registered for an M.Phil, than transferred to a Ph.D, but you could do the Ph.D in 3 years. I think the main point was, if you were doing badly, you had a good chance of coming out with an M.Phil, which is essentially a failed Ph.D in many cases - about not all.