Fix new prereleases detection

CHANGELOG.md

 # Change Log
 
+## [Unreleased]
+
+### Fixed
+
+- Fixed handling of duplicate dependencies with different constraints.
+- Fixed detection of new prereleases.
+
+
 ## [0.10.3] - 2018-06-04
 
 ### Fixed

poetry/packages/dependency.py

@@ -3,6 +3,7 @@ import poetry.packages
 from poetry.semver import parse_constraint
 from poetry.semver import Version
 from poetry.semver import VersionConstraint
+from poetry.semver import VersionRange
 from poetry.semver import VersionUnion
 from poetry.utils.helpers import canonicalize_name
 
@@ -34,6 +35,12 @@ class Dependency(object):
         self._pretty_constraint = str(constraint)
         self._optional = optional
         self._category = category
+
+        if isinstance(self._constraint, VersionRange) and self._constraint.min:
+            allows_prereleases = (
+                allows_prereleases or self._constraint.min.is_prerelease()
+            )
+
         self._allows_prereleases = allows_prereleases
 
         self._python_versions = "*"

poetry/version/version_selector.py

 from typing import Union
 
+from poetry.packages import Dependency
 from poetry.packages import Package
 from poetry.semver import parse_constraint
 from poetry.semver import Version
@@ -22,7 +23,7 @@ class VersionSelector(object):
         if target_package_version:
             constraint = parse_constraint(target_package_version)
         else:
-            constraint = None
+            constraint = parse_constraint("*")
 
         candidates = self._pool.find_packages(
             package_name, constraint, allow_prereleases=allow_prereleases
@@ -31,10 +32,12 @@ class VersionSelector(object):
         if not candidates:
             return False
 
+        dependency = Dependency(package_name, constraint)
+
         # Select highest version if we have many
         package = candidates[0]
         for candidate in candidates:
-            if candidate.is_prerelease() and not allow_prereleases:
+            if candidate.is_prerelease() and not dependency.allows_prereleases():
                 continue
 
             # Select highest version of the two