When /l/ and /n/ come together in a word, generally the /l/ will turn into /n/: so /ln/ becomes double /nn/.  Since /l/ is common ending, you will see this pattern whenever it is added to words ending in /n/.

su’nn        ‘cranberries’            (comes from su’n-l)        su’n    ‘cranberry’
signn        ‘socks’                    (comes from sign-l)        sign    ‘sock’

It also happens in the opposite direction: when you add /n/ to something ending in /l/.

etlatal            I am eating
etlatann        you are eating        (from etlatal-n)

mesgil            I am big
mesginn        you are big        (from mesgil-n)

This pattern is distinctive to Listuguj Mi’gmaq: speakers from the east do not make this sound change.  This is the reason, for example, why Listuguj speakers say /nnu/ while others say /lnu/.

(There is at least one important exception to this rule—seen in words like /nemulneg/ ‘we see you’—more on that when it comes up.)