In the spirit of the What is a Word™/Phrase™ series started by JLee, a special brand of Phrase™ and Word™ puzzles.
If a word conforms to a special rule, we call it a DD Word™.
Use the examples below to find the rule.
$$ \def\Pad{\P{0.0}} \def\Title{\textbf{ DD }} \def\S#1#2{\Space{#1}{20px}{#2px}}\def\P#1{\V{#1em}}\ \def\V#1{\S{#1}{9}} \def\T{\Title\textbf{Words}^™\Pad}\def\NT{\Pad\textbf{Not}\T\ }\displaystyle \smash{\lower{29px}\bbox[yellow]{\phantom{\rlap{rubio.2017.02.04}\S{6px}{0} \begin{array}{cc}\Pad\T&\NT\\\end{array}}}}\atop\def\V#1{\S{#1}{5}} \begin{array}{|c|c|}\hline\Pad\T&\NT\\\hline \text{JAMJAR}&\text{HONEYPOT}\\\hline \text{NOSTRIL}&\text{SINUS}\\\hline \text{GASOLINE}&\text{PETROLEUM}\\\hline \text{JASPER}&\text{TOPAZ}\\\hline \text{ENDNOTE}&\text{INDEX}\\\hline \text{PONDER}&\text{PERUSE}\\\hline \text{TRAMFUL}&\text{BEWILDER}\\\hline \end{array}$$
What is a DD Word™?
Hint 1:
Time for a hint!