According to most early decisors, the evening shema should be said after tseit hakokhavim (see mishna Berakhot 1:1 and its gemara), and the rishonim debate how to deal with a tsibbur that is praying maariv before that time. One oft-quoted suggestion in the name of Rav Hai Gaon (see Otzar HaGeonim) is to pray the maariv amida with the tsibbur and then say shema with its brakhot after tseit hakokhavim (without repeating the amida). Rav Hai Gaon then adds:
ואי חזי איניש לצלוי ראשונה בדהדי צבורא רשות ושניה חובה, שפיר דמי.
If it's possible for one to pray the first as an optional prayer and the second as a required prayer, even better.
That is, pray amida twice, first as an nedava prayer (I assume that this is what "optional" means, but if there are better suggestions, please profer them), and the second prayer-time (after dark) is a "normal" maariv (brakhot shema + shema + amida).
I am wondering how this last idea might work on Friday. On the one hand, it is forbidden to pray a nedava prayer on Shabbat or Yom Tov (see here), and saying a Shabbat/Y"T maariv amida seems to include accepting Shabbat on oneself at that time. On the other hand, perhaps the fact that it is not yet night means that this prohibition on nedava prayers on Shabbat/Y"T doesn't kick in. Additionally, none of the commentaries that I've seen that quote Rav Hai Gaon mention any difference on Shabbat — while one might expect some to omit mention of Shabbat/Y"T (since it may be "obvious" that it's not possible to do on erev Shabbat/Y"T), I'd expect at least a few commentaries to bring it up. Compounding the issue, there are versions of what Rav Hai Gaon said that switch around the order of the amidot: first the required amida and later the nedava prayer, when it's certainly fully Shabbat (see Talmidei Rabbeinu Yonah s.v. ומי, with girsaot) — how is this practice permitted?
Are there any sources that discuss this directly?