The first library is Phobos, probably `-unittest` switch is needed to use its local imports. This code should build fine:

m1.d:
---
struct S1 { } // should be before `f1`
void f1() { import std.ascii; }
---
m2.d:
---
struct S2 { } // should be before `f2`
void f2() { import std.ascii; }
---
Command (for Windows):
---
dmd -unittest -lib m1.d
dmd -unittest -lib m2.d m1.lib
---
m2.lib: Error: multiple definition of format_c_5b3: _D9format.1212__ModuleInfoZ and format: _D9format.1212__ModuleInfoZ
m2.lib: Error: multiple definition of format_c_5b3: __D9format.129__modtestFZv and format: __D9format.129__modtestFZv
---

This is at least major issue as it breaks building libraries and doesn't give any clue of its reason.