اِيْتَسَرَ → اِتْتَسَرَ → اِتَّسَرَ
The second example has root ي س ر, with original yāʾ as its first radical. Adjacent to the Form VIII tāʾ, yāʾ becomes tāʾ and assimilation follows.
The resulting doubled tāʾ is not evidence of a two-tāʾ root: one tāʾ represents the original yāʾ and the other is the added Form VIII consonant.
The same assimilation rule thus operates on a wāw-initial and on a yāʾ-initial root.
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Deriving اِتَّسَرَ from Yāʾ
Weak-Initial Roots: Elision, Substitution, Assimilation and Short Paradigms
اِيْتَسَرَ → اِتْتَسَرَ → اِتَّسَرَ
The second example has root ي س ر, with original yāʾ as its first radical. Adjacent to the Form VIII tāʾ, yāʾ becomes tāʾ and assimilation follows.
The resulting doubled tāʾ is not evidence of a two-tāʾ root: one tāʾ represents the original yāʾ and the other is the added Form VIII consonant.
The same assimilation rule thus operates on a wāw-initial and on a yāʾ-initial root.