Verbs with coalescence of consonants
Many verbs derive from Germanic strong verbs, which make many of their inflected forms through vowel gradation, as can be observed in Modern English patterns.
Some of these verbs have alternative regular forms, such as WEDDED and WETTED. (The verb hoist behaves similarly to verbs in this group, but this was originally itself a past form of the now obsolete verb hoise; similarly clad was originally – and sometimes still is – a past form of clothe.)
To find the correct form, you can use an online database of the irregular verbs.
For example: irregular verb COST