How would you explain software companies that are doing the same thing coexisting in the same market( horizontal/vertical )
I would expect that one of them is going to be better -> users choose the better product -> the one with the better product is going to have a monopoly over the market.
Examples of companies doing the same thing:
Website builders:
Wix , weebly
Web stores builders:
Wix , weebly , Shopify , Big Cartel , woocommerce
Help desk:
Zendesk , freshdesk , helpscout , groovehq
( not software but sill ):
Payment:
Stripe, Braintree
Tablets:
Ipad, Microsoft surface