If a line is simply named with a single letter, say for example $k$, is it incorrect to refer to this line as $\overleftrightarrow k$?
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$\begingroup$It's a popular idea that notations can be incorrect, but that's just wrong. They can be good or bad or standard or nonstandard, but "incorrect" doesn't really apply.
Certainly, bad notations are undesirable, especially if they cause confusion or mistakes. For that reason, it's usually good to stick to standard ones. But strictly speaking, you can choose whatever notation you think is clear to you and your audience.
Adding the double arrow over a letter for a line is certainly not confusing. However, in most cases I see lines, you usually just name it $L$ or $\ell$ and leave out the line.
$\endgroup$ $\begingroup$Sometimes, when it passes the certain $A$ and $B$, the line is denoted by $$\overleftrightarrow{AB}$$
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