The phrase "let go by" is typically spelled without an extra "t" (i.e. "let go by"), but some sources do offer the alternative spelling "letted go by". The latter spelling might make more sense phonetically, as the past tense of "let" could theoretically be pronounced "letted" (using the /ɛd/ sound to indicate past tense). However, "let" is often used in its past tense form with the regular "-ed" ending (i.e. "let go") rather than the irregular "letted".