Learn Farsi: ما در این سفر هم‌سفر بودیم. - We were travel companions on this journey.

It is made by adding the prefix هم- (ham-), which means same or together, to the noun سفر (safar), meaning journey or travel. Together, they literally mean same-journey, which translates to travel companion. This is a very common way to create nouns of mutual relationship in Farsi.

Since the subject is we (ما), shouldn't the word for travel companions be pluralized as هم‌سفران?

It can be, but it is not strictly necessary here. In Farsi, when a noun describes the state, role, or identity of a plural subject with the verb to be, it is often left in the singular form. Saying ما هم‌سفر بودیم is perfectly grammatically correct and natural.

The sentence uses the root word سفر twice right next to each other. Does that sound repetitive to a native speaker?

Not at all. While English usually avoids repeating the same root word closely (we translate it as travel companions on this journey rather than travel companions on this travel), Farsi is much more comfortable with this kind of repetition. The phrasing sounds completely natural.

Why is there a small gap in the middle of the word هم‌سفر?

That is a half-space, used in Farsi typing to keep the prefix هم- visually distinct from the root word سفر without breaking them into two separate words. While you might sometimes see it written fully connected as همسفر in casual handwriting, using the half-space is the standard modern spelling rule for compounds with this prefix.