Pressing it for the second time is OK on v6, but not in v7.3.1. Instead od pressing twice i have to press and keep tab key down for more that 1-2 seconds.If you've typed just the common part, pressing the tab key once has no effect. However, pressing it for the second time shows all possible completions in compact form:
/system/[TAB] is OK - shows all possibilities
/system/de[TAB][TAB] is OK on v6, doesn't work on v7 (two possibilities starts with "de")
/system/def[TAB] is OK completes the only one possibilities