Can fortify position give its effect twice? So shield 2 in one combat and then another combat happens to give it shield 2 again so +4

No. Fortified Position's Shield 2 lasts only "this combat," so the first combat's Shield expires the moment that combat ends (Rule 466.7.c). The next combat's trigger gives a fresh Shield 2, but the old one is already gone — the unit ends up with Shield 2 (+2 Might), not Shield 4.

Two things prevent the doubling:

• Cross-combat stacking doesn't happen — "this combat" effects all expire simultaneously at the end of combat (Rule 466.7.c), so the two grants never coexist.
• Fortified Position can't fire twice in one combat either — defend triggers are only checked once per combat (Rule 383.4.f.2.a).

Where Shield does stack is when sources overlap in the same combat — e.g., a unit with native Shield 2 defending on Fortified Position totals Shield 4 (FAQ 2459). But that requires two simultaneous grants, not one per combat.