Inquiries in a short period of time will affect your score, again the number of points will vary by each specific situation. Generally., better scores will be less affected than lower scores, but YMMV. The credit scoring systems were not designed to factor in reward and churn behavior where someone is not actually seeking credit but miles. Overall, that population., although it seems large on this forum, is a tiny. More inquiries across a number of products in a short time frame (60-90 days) will definitely trigger a change in score. Duplicate checks by individual companies will not, as that info is not sent to the credit bureaus

Just a note to add to this thread. I applied for a Chase United MP card in Dec and was approved after having to go through the Executive Services office. Then I saw the Continental offer in Feb and applied, was denied (had a BK almost ten years ago, just 4 mos to go and its gone off my CR) (bad divorce caused this) anyway, called the Executive Offces again thinking yeah they should give it to me like they did in Dec, well no luck, denied as they said I didn't have a long enough history with them, fico is 750 only 4 inquiries in 2 years and 2 of them with Chase. Anyway didn't like this decision wrote a letter to the CEO asking for a another review, well the same Exec Service lady wrote to me reiterating her position (so much for another reviewer) and then a week later they dropped my credit limit from $5k to $900 bucks. They said it was a review and they determined I have to much credit. Only 9 cards open with no balances on them! Well it was just payback for trying to get someone to look at the whole picture. I make good income, no credit problems in 10 years, never late, etc. Well I called the CSR number and spoke to a credit manager and he raised the limit back, noting that he didn't see whey they even did a review. I told him what took place and he said Sorry I can't comment on that, but it is fixed. Can't stand Chase. Will be cancelling their card as the earliest date. They don't treat the customers so good. Still got the Cont points through my wife and sons. (150,000 miles though)