For รร (such as in บรรพชา or กรรณ), I think you may call it double letter. In this case, รร works as a vowel. But for ญญ in สัญญา, I think you should look at them separately. The first ญ works as an ending consonant and sounds like "n" in English; while the second ญ is an initial consonant, so its sounds is equal to "y" in English. It actually has nothing to do with 2 ญs being together. In a normal case, when being alone, ญ as an ending consonant always pronounces as "n" (e.g. จรัญ, อาสัญ); and ญ as an initial consonant always pronounces as "y" (e.g. ญาติ, ญัตติ) However, there are times that only one ญ has 2 roles: both ending (for the previous syllable) and initial (for the next syllable). In those cases, it pronounces as "n" then "y", such as อัญมณี