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Old June 18, 2018, 08:36 AM
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Rusty Rusty is offline
Señor Speedy
 
Join Date: Aug 2007
Location: USA
Posts: 11,299
Native Language: American English
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Native speakers from every clime and continent make mistakes when speaking.

I've heard the same omission you have, several times. In fact, you'll find that there are those who write the structure with the preposition in parentheses when teaching the construct to others, online.

Here is what the Diccionario panhispánico de dudas site says on the subject:

estar seguro. ‘No tener duda’. La persona o cosa sobre la que no se tiene duda se expresa mediante un complemento introducido por de: «Estoy segura de que la oirán» (Padilla Jardín [Cuba 1981]). En el habla esmerada, no debe suprimirse la preposición: Estoy seguro que.
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