I've searched all options in Line format, and I can make it dotted or change the arrow heads, but it seems impossible in my LibreOffice 5.0.3.2 to draw something as simple as a double line for an arrow, to get something like
How to format an arrow as a double line in LibreOffice Draw
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There is no option for double lines found readily, as LibreOffice 5.3.What about custom line stylesThis article on LibreOffice Help has noted how to use custom line style from target file.To load a line styles file:Choose Format - Line, and then click the Line Styles tab.Click the Load Line Styles button.Locate the file containing the line styles that you want to load, and then click OK. The file has the format [filename].sod.To save a line styles file, click the Save Line Styles button, enter a filename, and then click OK.While .sod file seems to be useful for saving and loading modified line styles, there is no relevant information to create one from new.Create arrows using workaroundsThere are several workarounds to create an arrow with double lines. The following screenshot shows how the arrows would look like.See the following steps for respective workarounds.Workaround A (Easy)Use Unicode character that is found in Unicode Block Arrows. For example, there is U+21D2 RIGHTWARDS DOUBLE ARROW that represents  character.In Drawing toolbar, select Text Box and draw a text box.Type the Unicode character: Press key combination Ctrl+Shift+U, then press 2 1 D 2 and finally press Enter. This will insert .Finally, adjust size of the character text font and text box.Workaround B (Flexible)Use several drawing objects and merge together to reproduce the arrow with desired style. The following steps will explain for arrow with double lines.In Drawing toolbar, select Line ends with arrow and draw the line; Keep the arrow small, such that only the arrow end is visible and the line is hidden underneath.Select the object, Format (or right-click menu)  Lines... Line tab - Line Properties, then change Width to 0.04; under Arrow Styles, change End style to Arrow Concave and arrow width to 0.30 (the order of changing is important).In Drawing toolbar, select Insert line and draw the line; The length of line is user's choice.Select the object, Format (or right-click menu)  Lines... Line tab - Line Properties, then change Width to 0.04.Make a copy of line from step 4.Place the lines appropriately with respect to the arrow.Select all objects, then Modify (or right-click menu)  Shapes Merge.Additional notesUsing above workarounds, there is one significant drawback: resize does not work like built-in objects. For example, horizontal resizing of the merged object will cause the arrow end to be stretched instead of only the line becomes longer. Therefore, to have a longer line, the length should be adjusted beforehand (Workaround B, step 3).Unlike Group action that groups objects, the Merge action creates merged object that can be resized as whole. However, the only way to revert Merge action is Undo or Ctrl+Z (there is no Unmerge menu) at time of edit.Above all, users are advised to make several copies of objects before doing the Merge action, in case the end result becomes unexpected (at point which there are too many steps to Undo).Tested workarounds in LibreOffice 5.3 in Xubuntu 14.04.