different double checking protocol in the past?

[ixfd64]

I know that an exponent must be tested at least twice in order to be considered "verified."

However, the PrimeNet page also says this:

Double-check assignments are identical to Lucas-Lehmer assignments, however a (D) also appears to the right of the exponent. These comparatively rare tests recompute earlier residues of questionable reliability.

Were double checks "unnecessary" in GIMPS' early days or something?