A common, and untrue conspiracy theory.
While there is a small hint of truth, that the voting numbers will not reflect actual support in a FPTP system the idea that that is reflective of the chance of a third party actually winning or that it favors the **same** two parties every time is absolutely incorrect.
FPTP only gives the illusion of a two party system because the parties that are in the lead tend to take a larger portion of the vote than the **real** support (due to effects from FPTP, namely coalition building effects). This however in no way suggests the same two parties will be locked in election after election. In fact quite the opposite.
FPTP actually will show a pattern where every time a third-party option wins it will go from very low support in the previous election (ie 1%) to majority support int he next, in a single step (51%+). Essentially FPTP only ensures every election 2 parties will take the bulk of the votes, it in no way suggests those 2 parties are more likely to remain in control.
It helps when you look at real support and how it translates. If you have real support such as Party A:21% B: 20% C: 19% D: 15% E: 15% F: 10% Then the actual voting results will be something like A: 60% B: 39%. So all C really needs is 2% more support to change things next election in all reality and if they do then youd have A and C getting the lead.