The sunni perspective says one my redo istikhara, as often as one wants -I found no clear evidence for that except for the vague wording of the related hadith-. I have not found any limitation so far, especially for the case one had a bad or negative result. So maybe this part needs some further research.
There are two supporting evidences for this practice however:
In sahih Muslim:
...When he prayed, he prayed thrice, and when he asked for God's blessings, he asked thrice. Then he said thrice: ...
and a da'if hadith where the prophet() might have recommended his companion Anas () to repeat istikhara seven times.
This fatwa on islamqa #11981 offers many answers and details on the topic of istikhara from sunni perspective.
The shi'a perspective according the marji' al-Sistani is quoted in a few answers here on www.sistani.org - My own translation take it carefully- in the Q&A 1, 4:
السؤال: هل يجب علي المستخير الالتزام بالاستخارة؟ وهل يجوز له اعادة الخيرة بعد مضي وقت؟
Question: Does the person doing istikhara needs to abide to the result? And is it allowed to re-do istikhara after a time gap.
الجواب: ينبغي العمل بموجبها ولا تعاد الا بتبديل الموضوع ولوبالصدقة.
Answer: One is asked to abide to the result and shouldn't repeat it, only if one changed the reason or situation, for example by a sadaqah.
In fatwa or Q&A #11 he explains that the sadaqah is meant to be done before a second Istikhara, if the first ended with no clear result, and in:
السؤال: هل يجوز الأستخارة على نفس الموضوع اكثر من مرة بغية الحصول على نتيجة ايجابية وبأيها يؤخذ؟ (اي بالأولى ام بألاخيرة)؟
Question: Is repeating istikhara for the same "Topic" more than once with the intention of gaining a positive result allowed, and if so what result should one follow? (The first or last?)
الجواب: لا معنى لتجديد الإستخارة لموضوع واحد إلاّ اذا تغيّرت بعض الظروف أو مضى زمان بحيث يحتمل تغّير المصالح والمفاسد.
Answer: There's no sense in repeating Istikhara for the same reason, only if the circumstances have changed and time has passed in a way that one may suppose that the benefits and abuses have changed
Note that in the fatwas or Q&A #12 and #13 addressing not abiding to Isitkhara al-Sistani allows it in case of a good/positive result (#12) and in case of a bad result (#13) he allows a sadaqah by dhabihah (zabihah), saying that this might not change the what was prescribed for that person in case of not abiding to the Istikhra result.
So according to shi'a view repeating Istikhara needs a change of the circumstances after a certain time, else it is meaningless.
Differences in Term or practice
Also note that even if the topic of Istikhara and the purpose seems to be the same for both sunni and shi'a there are clear differences, while sunni have clear and specific ally formulated du'a which is performed after an optional prayer. Shi'a have other options as I could conclude on the answers about Istikhara or Khiyara of the marji' al-Sistani, for example doing Istikhara with a-sabaha (Q&A 6), Isitkhara is only recommended if there is a doubt and one has nobody to ask for guidance on the matter(Q&A 7, 8,9) (sunni's recommend it, but one still may do Istikhara nevertheless), Istikhara via qur'an and the prayer of it which seems to be called the prayer Patches or Palm leaves صلاة الرقاع where one may write on six "papers" (or equal) 3 times "do it" and another 3 times "don't do it" ...