[sjoerd_visscher]

Nope, there's just one: look at column 1 and 4, so far they are the same.

[timv]

That's the point.

The rules don't explicitly say that columns need to be unique (only rows).

Within the stated rules there are (as best I can see) 2 valid solutions to that puzzle. It is the introduction of the unstated unique column rule that resolves it.

[judk]

The author is not a native English speaker, but it seems obvious that the row rule should apply to columns.