It's pretty obvious what the most well-known trademark is. I don't think Google would be giving ibm.dev to the International Brotherhood of Magicians [1]. Here's an example as it pertains to domain names [2]. Of course Instagram is different and there's much less oversight, but they would probably do something similar, where they first verify a trademark, and if multiple parties have the same word in the trademark, determine which.

"2.5 Does putting my trademark in the Clearinghouse mean that I automatically get my trademark as a domain name in all new gTLDs?

No. The Clearinghouse verifies and maintains information from many jurisdictions and classes of goods or services, and many parties may have legitimate rights in the same trademark. Allocation of domain names in a particular TLD occurs according to the registry policies for that TLD."

1: http://tmsearch.uspto.gov/bin/showfield?f=doc&state=4806:9gr...

2: https://newgtlds.icann.org/en/about/trademark-clearinghouse/...

I didn't say that they would be infringing on Apple, Inc's trademark and I don't think that. I only think that in disputed ownership, based on instagram's purpose of providing a communication platform, the rightful owner would be Apple, Inc, not someone who happened to register it first. And they have a way of determining who the rightful owner, to them, is. Clearly the definition of rightful owner isn't universal, nor is it 100% consistently determined, but a process can be made for determining it. Here's an example: https://newgtlds.icann.org/en/about/trademark-clearinghouse/...