Is the phrase "a stark contrast" redundant?

There is no redundancy in 'stark contrast' as in typical cases of redundancy.

Contrast is an attribute with degrees of variation: none to total.

stark in stark contrast merely qualifies the attribute to the level of extreme perceptibility.

Two things may be different to some extent. The difference is not perceptible so long as the contrast is below a certain threshold. Beyond that, and then a little further, the difference becomes so "glaring" that it comes out prominently: 'stark contrast'.

You could argue that stark contrast is redundant. However, it is perfectly acceptable to use redundancy or repetition for emphasis. This is exactly what is being done in this somewhat ideomatic expression. If stark contrast is leaning too far towards cliché for you then you could substitute another intensifier for stark, but then you lose the benefit of clarity that comes with a well established expression.