Yes. An equity stake will almost certainly be diluted.

Successful startups host many rounds of financings, all the way to an IPO. For each financing, the startup issues additional stock to the new investors. This is healthy and normal as long as the company's value increases with each funding round. For example, the first investor in Facebook, Peter Thiel, originally purchased ~10% of the company for $500,000. By 2011, that stake was diluted to under 3% but estimated to be worth ~$2 billion.

Sometimes, when things are not going well, the startup can go bankrupt or raise more money in a "down round," which means the company's value has decreased since the last financing. This is very bad for the founders and past investors; the dilution happens rapidly. But it's preferable to the startup going bankrupt and the investors losing everything.