Texture border

I render a checkboard texture (white & black squares) into a square. I applay the GL_REPEAT params to duplicate the texture accross the square. I set the appropriate params to render it exactly twice accross the square.
Result: I see a grey border to the square (I guess: the black pixel in the right side interpolates with the white pixel on the left side).
How can I prevent this artifact?
Thanks,
Yossi