The term "show by double line" is often used in legal documents and contracts to indicate a specific type of emphasis or notation. Some synonyms for this phrase might include "highlight with a double line," "emphasize with a double underline," or "underline with two lines for emphasis." These terms all convey the same general idea of adding extra weight or importance to a specific portion of text. Other possible synonyms for "show by double line" might include "call out with a double border," "mark with double rulers," or "draw attention to with two lines." Whatever the phrasing, the goal is always to ensure that the reader understands the significance of the highlighted section.