Rule 1: 1. The imperative سَمِّ and the imperatives of doubled verbs
سَمَّى يُسَمِّي to name, and in particular to say bismillāh, is Form II of the root س م و: its last radical is weak, not doubled. In the jussive the final ي drops (لَمْ يُسَمِّ) and the imperative is likewise سَمِّ, feminine سَمِّي, plural سَمُّوا. The shadda belongs to the Form II pattern. A true doubled verb also ends its imperative in a shadda, but for a different reason, the contraction of two identical radicals, and Form I therefore has three possible forms: مُدَّ, مُدِّ or the uncontracted اُمْدُدْ stretch out!; كُفَّ or اُكْفُفْ desist!; عُدَّ or اُعْدُدْ count!. Before an ending that begins with a vowel there is only one form: مُدِّي, مُدُّوا. Derived forms: أَعِدَّ prepare! (IV), اِسْتَعِدَّ get ready! (X).
سَمِّ اللّٰهَ وَكُلْ بِيَمِينِكَ وَكُلْ مِمَّا يَلِيكَ.
sammi llāha wa-kul bi-yamīnika wa-kul mimmā yalīka
Say the name of God, eat with your right hand, and eat from what is nearest you.
The Prophet's ﷺ instruction to a boy at the table (Bukhārī and Muslim); سَمِّ is the imperative of سَمَّى.
سَمُّوا وَابْتَدِئُوا بِالطَّعَامِ.
sammū wa-btadiʾū bi-ṭ-ṭaʿāmi
Say bismillāh and begin the meal.
Plural سَمُّوا; the weak radical disappears before ـُوا.
مُدَّ يَدَكَ إِلَى مَا أَمَامَكَ.
mudda yadaka ilā mā amāmaka
Stretch out your hand to what is in front of you.
Imperative of the doubled مَدَّ يَمُدُّ; also مُدِّ or اُمْدُدْ.
كُفَّ عَنِ الْأَكْلِ إِذَا شَبِعْتَ وَلَا تَقُمْ قَبْلَ الْجَمِيعِ.
kuffa ʿani l-akli idhā shabiʿta wa-lā taqum qabla l-jamīʿi
Stop eating when you are full, and do not rise before everyone.
كُفَّ from كَفَّ يَكُفُّ; لَا تَقُمْ jussive of the hollow قَامَ.
أَعِدِّي الطَّعَامَ لِثَمَانِيَةٍ يَا أُمَّ خَالِدٍ.
aʿiddī ṭ-ṭaʿāma li-thamāniyatin yā umma khālidin
Prepare food for eight, Umm Khālid.
Feminine imperative of أَعَدَّ (Form IV); only one form before ـِي.
اِسْتَعِدُّوا، فَالضُّيُوفُ عَلَى الْبَابِ.
istaʿiddū, fa-ḍ-ḍuyūfu ʿalā l-bābi
Get ready, the guests are at the door.