I am aware of the fundamental theorem of algebra, i.e., the degree of a polynomial is the number of roots of the polynomial. For example, $x^2 - 9 = 0$ would have two solutions: $x=3$ and $x=-3$. However, sometimes I come across quadratic polynomials that only have one root, e.g.,
$$t^2 - 2 t + 1 = (t-1)(t-1) = 0$$
which only has the solution $1$, or so I think. Is there some underlying concept that I am overlooking?